delivered the opinion of the Court.

The issue before us is ah important one for the people of our country and their Government. It involves the delicate question of the President’s power, acting through the Attorney General, to authorize electronic surveillance in internal security matters without prior judicial approval. Successive.Presidents for more, than one-quarter of a century have authorized such surveillance in varying degrees,1 without guidance from the Congress or a definitive decision of this Court.. This case brings the issue here for the first time. Its resolution is a matter of national concern, requiring sensitivity both to the Government’s right to protect itself from unlawful subversion and attack and to the citizen’s right to be secure in his privacy against unreasonable Government intrusion.

This case arises from a criminal proceeding in the United States District Court for the Eastern District of Michigan, in which the United States charged three defendants with conspiracy to destroy Government property in violation of 18 U. S. C. § 371. One of the defendants, Plamondon, was charged with the dynamite bombing of an office of the Central Intelligence Agency in Ann Arbor, Michigan.

During pretrial proceedings, the defendants moved to compel the United States to disclose certain electronic *300surveillance information and to conduct a hearing to determine whether this information “tainted” the evidence on which the indictment was based or which theGovernment intended to offer at trial. In response, the- Government filed an affidavit of the Attorney General, acknowledging that its agents had overheard conversations in which Plamondon had participated. The affidavit also stated that the Attorney General approved the wiretaps “to gather intelligence information deemed necessary to protect the nation from attempts of domestic organizations to attack and subvert the existing structure of the Government.” 2 The logs of the surveillance *301were filed in a sealed exhibit for in camera, inspection by the District Court. •

On the basis of the Attorney General’s affidavit and the sealed exhibit, the' Government' asserted that the surveillance was lawful, though conducted without prior judicial approval, as a reasonable exercise of- the President’s power (exercised through the Attorney General) to protect the national security. The District Court held that the surveillance violated the Fourth . Amendment, and ordered the Government to 'make full, disclosure to Plamondon of his overheard conversations. 321 F. Supp. 1074 (ED Mich. 1971).

The Government then filed in the Court of Appeals for the Sixth Circuit a petition for-a writ of mandamus to set aside the District Court order, which was stayed pending final disposition of the case. ..After- concluding that it had jurisdiction,3 that ‘court held that the surveillance was unlawful and that the District Court had properly .required disclosure of the overheard conversations, 444 F. 2d 651 (1971). We granted certiorari, 403 U. S. 930.

I

Title III of the Omnibus Crime Control and Safe Streets Act, 18 U. S. C. §§ 2510-2520, authorizes the use of electronic surveillance for classes of crimes care*302fully specified in 18 U. S. C. § 2516. Such surveillance is subject to prior court order. Section 2518 sets forth the detailed and particularized application necessary to obtain such an order as well as carefully circumscribed conditions for its use. The Act represents a comprehensive attempt by Congress to promote more effective control of crime while protecting the privacy of individual thought and expression. Much of Title III was drawn to meet the constitutional requirements for electronic surveillance enunciated by this Court in Berger v. New York, 388 U. S. 41 (1967), and Katz v. United States, 389 U. S. 347 (1967).

Together with the elaborate surveillance requirements in Title III, there is the following proviso; 18 U. S. C. §2511(3):

“Nothing contained in this chapter or in section 605 of the Communications Act of 1934 (48 Stat. 1143; 47 U. S. C. 605) shall limit the constitutional power of the President to take such measures as he deems necessary to protect the Nation against actual or potential attack or other hostile acts of a foreign power, to obtain foreign' intelligence information deemed essential to the security of the United States, or to protect national security information against foreign intelligence activities. Nor shall anything contained in this chapter he deemed to limit the constitutional power, of the President to take such measures as he deems necessary to protect the United States against the overthrow of the Government by force or other unlawful means, or against any other clear and present danger to the structure or existence of the Government. The contents of any wire or oral communication intercepted by authority of the President in the exercise of the foregoing powers may be received in evidence in any trial hearing, *303or other proceeding only where such interception was reasonable, and shall not be otherwise used or disclosed except as is necessary to implement that power.” (Emphasis supplied.)

The Government relies on § 2511 (3). It argues that “in excepting national security surveillances from the Act’s warrant requirement. Congress recognized the President’s authority to conduct such, surveillances without prior, judicial approval.” Brief for United States 7, 28. The section thus is viewed as a recognition or affirmance of a constitutional authority in the President to conduct warrantless domestic security surveillance such as that involved in this case.

We think the language of § 2511 (3), as well as the legislative history of the statute, refutes this-interpretation. . The relevant language is that:

“Nothing contained in this chapter . . . shall limit the constitutional power of the President to take such measures ás he deems necessary to protect.. .”

against the dangers specified. At most, this is an implicit recognition that the President does have certain powers in the specified areas. Few would doubt this, as the section refers — among other things — to protection “against actual or potential attack or other hostile acts of a foreign power.” But so far as the use of the President’s electronic surveillance power is concerned, the language is essentially neutral.

Section 2511 (3) certainly confers no power, as the language is wholly inappropriate for such a purpose. It merely provides that the Act shall not be interpreted to limit or disturb such power as the President may have under the Constitution. In short, Congress simply left presidential powers where it found thém. This view is reinforced by the general context of Title III. Section 2511 (1) broadly prohibits the use of electronic *304surveillance “{ejxcept as otherwise specifically provided in this chapter.” Subsection (2) thereof contains four specific exceptions. In each of the specified exceptions, the statutory language is as follows:

“It shall not be unlawful ... to intercept” the particular type of communication described.4

The language of subsection (3), here involved, is to be contrasted with the language of the exceptions set forth in the preceding subsection. Rather than stating that warrantless presidential uses of electronic surveillance “shall not be unlawful” and thus employing the standard language of exception, subsection (3) merely disclaims any intention to “limit the constitutional power of the President.”

The express grant of authority to conduct surveil-lances is found in § 2516, which authorizes the Attorney General to make application to a federal judge when surveillance may provide evidence of certain offenses. These offenses are described with meticulous care and specificity.

Where the Act authorizes surveillance, the procedure to be followed is specified in §2518. Subsection (1) thereof requires application to a judge of competent jurisdiction for a prior order of approval, and states in detail the information réquired in such application.5 *305Subsection (3) prescribes the necessary eleménts of probable cause which the .judge must find before issuing an order authorizing an interception. Subsection (4) sets forth the required contents of such an order.. *306Subsection (5) sets strict time limits on an order. Provision is made in subsection (7) for “an emergency-situation” found to exist by the Attorney General (or by the principal prosecuting attorney of a State) “with respect to conspiratorial activities threatening the national security interest.” In such a situation, emergency surveillance may be conducted “if an application for an order approving the interception is made . . . within forty-eight hours.” If such an order is not obtained, or the application therefor is denied, the interception is deemed to be a violation of the Act.

In view of these and other interrelated provisions delineating permissible interceptions of particular criminal activity upon carefully specified conditions, it would have been incongruous for Congress to have legislated with respect to the important and complex area of national security in a single brief and nebulous paragraph. This would not comport with the sensitivity of the problem involved or with the extraordinary care Congress exercised in drafting other sections of the Act. We therefore think the conclusion inescapable that Congress only intended to make clear that the Act simply did not legislate with respect to national security surveillances.6

The legislative history of §2511(3) supports this interpretation. Most relevant is the colloquy between Senators Hart, Holland, and McClellan on the Senate floor:

“Mr.'HOLLAND. . .. The section [2511(3)] from which the Senator [Hart] has read does not affirma*307tively give any power. ... We are not affirmatively conferring any power upon the President. We are simply saying that nothing herein shall limit such power as the President has under the Constitution. . . . We certainly do not grant him a thing.

“There is nothing affirmative in this statement.

“Mr. McCLELLAN. Mr. President, we make it understood that we are not trying to take anything away from-him.

“Mr. HOLLAND. The Senator is correct.

“Mr. HART. Mr. President, there is no intention • here to expand by this language a constitutional power. Clearly we could not do so.

“Mr. McCLELLAN. Even though intended, we could not do so.

“Mr. HART. . . . However, we are agreed that this language should not be regarded as intending to grant any authority, including authority to put a bug on, that the President does not have now.

“In addition, Mr. President, as I think our exchange makes clear, nothing in section 2511 (3) even attempts to define the limits of the. President’s national security power under present law, which I have always found extremely vague .... .Section 2511(8) 'merely says that if the President has such a power, then its exercise is in no way affected by title III.” 7 (Emphasis supplied.)

*308One could hardly expect a clearer expression of congressional neutrality. The debate above explicitly indicates that nothing in § 2511 (3) was intended to expand or to contract or to define whatever presidential surveillance powers existed in matters affecting the national security. If we could accept the Government's characterization of § 2511 (3) as a congressionally prescribed exception to the general requirement of a warrant, it would be necessary to consider the question of whether the surveillance in this casé came within the exception and, if so, whether the statutory exception was itself constitutionally valid. But viewing § 2511 (3) as a congressional' disclaimer and expression of neutrality, we hold that the statute is not the measure of the executive authority asserted in this case. Rather, we must look to the constitutional powers of the President.

II

It is important at the outset to emphasize the limited nature of the question before the Court. This case raises- no constitutional challenge to electronic surveillance as specifically authorized by Title III of the Omnibus Crime Control and Safe Streets Act of 1968. Nor. is there any question or doubt as to the necessity of obtaining a warrant in the surveillance of crimes unrelated to the national security interest. Katz v. United States, 389 U. S. 347 (1967); Berger v. New York, 388 U. S. 41 (1967). Further, the instant case requires no judgment on the scope of the President's surveillance power with respect to the activities of foreign powers, within or without this country. The Attorney General’s affidavit in this case states that, the surveillances were *309“deemed, necessary to protect the nation from attempts of domestic organizations to attack and subvert the existing structure of Government” (emphasis supplied). There -is no evidence of any involvement, directly or indirectly, of a foreign power.8

Our present inquiry, though important, is therefore a narrow one. It addresses a question left open by. Katz, supra, at 358 n. 23:

“Whether safeguards othér than prior authorization by- a magistrate would satisfy the Fourth Amendment in a situation involving the national-security . . 7 .”

The determination of this question requires the essential Fourth .Amendment inquiry into .the “reasonableness” of the search and seizure in question, and the way in which that “reasonableness” derives content and mean*310ing through reference to the warrant ulause. Coolidge v. New Hampshire, 403 U. S. 443, 473-84 (1971).

We begin the inquiry by noting that the President of the United States has the fundamental duty, under Art. II, § 1, of the Constitution, to “preserve, protect and defend the Constitution of the United States.” Implicit in that duty is the power to protect our Government against those who. would subvert or overthrow it by unlawful means. In the discharge of this duty, the President — through the Attorney General— may find it necessary to employ electronic surveillance to obtain intelligence information on the plans of those who plot unlawful acts against the Government.9 The use of such surveillance in internal security cases has been sanctioned more or less continuously by various Presidents and Attorneys General since July 1946.10 *311Herbert Brownell, Attorney General under President Eisenhower, urged the use of electronic surveillance both in internal and international security matters on the grounds that those acting against the Government

“turn to the telephone to carry on their intrigue. The success of their plans frequently rests upon piecing together shreds of information received from many sources and many- nests. The participants in the conspiracy are often dispersed and stationed in various strategic positions in government and industry throughout the country.”11

Though the Government and respondents debate their seriousness and magnitude, threats and acts of sabotage against the Government exist in sufficent number to justify investigative powers with respect to them.12 The covertness and complexity of potential unlawful con*312duct against the Government and the necessary dependency of many conspirators upon the telephone make electronic surveillance an effective investigatory instrument in certain circumstances. The marked acceleration in technological developments and sophistication in their use have resulted in new techniques for the planning, commission, and concealment of criminal activities. It would be contrary to the public interest for Government to deny to itself the prudent and lawful employment of those very techniques which are employed against the Government and its law-abiding citizens.

It has been said that “[t]he most basic' function of any government is to provide for the security of the individual and of his property.” Miranda v. Arizona, 384 U. S. 436, 539 (1966) (White, J., dissenting). And unless Government safeguards its own capacity to function and to preserve the security of its people, society itself could become so disordered that all rights' and liberties would be endangered. As Chief Justice Hughes reminded us in Cox v. New Hampshire, 312 U. S. 569, 574 (1941):

“Civil liberties, as guaranteed by the Constitution, imply the existence of an organized society maintaining public order without which liberty itself would be lost in the excesses of unrestrained' abuses.”

But a recognition of these elementary truths does not make the employment by Government of electronic surveillance a welcome development — even when employed with restraint' and under judicial supervision. There is, understandably, a deep-seated uneasiness and apprehension that this capability will be used to intrude upon cherished privacy of law-abiding citizens.13 We *313look to the Bill of Rights to safeguard this privacy. Though physical entry of the home is the chief evil against which the wording of the Fourth Amendment is directed, its broader spirit, now shields private speech from unreasonable surveillance. Katz v. United States, supra; Berger v. New York, supra; Silverman v. United States, 365 U. S. 505 (1961). Our decision in Katz refused to lock the Fourth Amendment into • instances of actual physical trespass. Rather, the Amendment governs “not only the seizure of tangible items, but extends'as well to the recording of oral statements . . . without any ‘technical trespass under . . . local property law.’ ” Katz, supra, at 353. That decision implicitly recognized that the broad and unsuspected governmental incursions into conversational privacy which electronic surveillance entails14 necessitate the application of Fourth Amendment safeguards.

National security cases, moreover, often reflect a convergence of First and Fourth Amendment values not present in cases of “ordinary” crime. Though the investigative duty of the executive may be stronger in such cases, so also is there greater jeopardy to constitutionally protected speech. “Historically the struggle for freedom of speech and press in England was bound up with the issue of the scope of the search and seizure *314power,” Marcus v. Search Warrant, 367 U. S. 717, 724 (1961). History abundantly documents the tendency of Government — however benevolent and benign its motives — to view with suspicion those who most fervently dispute its policies. Fourth Amendment protections become the more necessary when the targets of official surveillance may be those suspected of unorthodoxy in their political beliefs. The danger to political dissent is acute where the Government attempts to act under so vague a concept as the power to protect “domestic security.” Given the difficulty of defining the domestic security, interest, the danger of abuse in acting to protect that interest becomes apparent. Senator Hart addressed this dilemma in the floor debate on § 2511 (3):

“As I read it — and this is my fear — we are saying that the President, on his motion, could declare— name your favorite poison — draft dodgers, Black Muslims, the Ku Klux Klan, or civil rights activists to be a clear and present danger to. the structure or existence of the Government.” 15

The price of lawful public dissent must not be a dread of subjection to an unchecked surveillance power. Nor must the fear of unauthorized official eavesdropping deter vigorous citizen.dissent and discussion of Government action in private conversation. For private dissent, no less than open public discourse, is essential to our free society.

Ill

As the Fourth Amendment is not absolute in its terms, our task is to examine and balance the basic values at stake in this case: the duty of Government *315to protect the domestic security, and the potential danger posed by unreasonable surveillance to individual privacy and free expression. If the legitimate need of Government to. safeguard domestic security requires the use of electronic surveillance, the question is whether the needs of citizens for privacy and free expression may not be better protected by requiring a warrant before such surveillance, is undertaken; We must also ask whether a warrant requirement would unduly frustrate; the. efforts of Government to protect itself from acts of subversion and overthrow, directed against it.

Though the Fourth Amendment speaks broadly of ‘unreasonable searches and seizures,” the definition of “reasonableness”' turns, at least in part, on the more specific commands of the warrant clarise. Some have argued that “[t]he relevant'.test is hot whether it is reasonable to procure á search warrant, but whethér the search was reasonable,”. United States v. Rabinowitz, 330 U. S. 56, 66 (1950).16 This view, however, overlooks the second clause of the Amendment. The warrant clause of the Fourth Amendment is not dead language. Rather, it has been.

“a valued part of our constitutional law for decades, and it has determined the result in scores and scores' of cases in courts all over this country. It is riot an inconvenience to be somehow ‘weighed’ against the claims of, police efficiency. It is, or should *316-be,- an- important working part of our machinery of government, operating as a matter of course to check the - ‘well-intentioned but mistakenly overzealous executive officers’. who are a part of any system of law enforcement.” Coolidge v. New Hampshire, 403 U. S., at 481.

See also United States v. Rabinowitz, supra, at 68 (Frankfurter, J.,, dissenting); Davis y. United States, 328 U. S. 582, 604 (1946) (Frankfurter, J., dissenting):

Over two centuries ago, Lord Mansfield held that common-law principles prohibited warrants that ordered the' arrest of unnamed individuals who the. officer might conclude were guilty, of seditious libel. “It is not fit,” . said Mansfield, “that the receiving. oi- judging of the. information should be left to the discretion of . the. officer. The magistrate ought to . judge;’ and should . give certain directions to the officer.” Leach v. Three of the King’s Messengers, 19 How. St. Tr. 1001, 1027 (1765).

• Lord Mansfield’s formulation touches' the very heart of the Fourth Amendment directive: that, where practical, a governmental search,and seizure should repre- ' sent both the efforts of the officer to gather evidence of .wrongful acts and the judgment of the magistrate that the collected evidence is sufficient to justify invasion of a citizen’s private premises or conversation.' Inherent in the concept of a warrant is its issúance by a “neutral and detached, magistrate.” Coolidge v. New Hampshire, supra, at 453; Katz v. United States, supra, at 356. The further requirement of “probable cause” instructs the magistrate that baseless searches shall not proceed.

These Fourth Amendment freedoms cannot properly be guaranteed if domestic security surveillances may be' conducted solely .within the discretion of the Execu*317tive Branch. The Fourth Amendment does not contemplate the executive officers of .Government as neutral and disinterested magistrates. . Their duty and responsibility are to enforce the- laws, to investigate, and to prosecute. Katz v. United States, supra, at 359-360 (Douglas, J., concurring). But those charged with this investigative and prosecutorial duty should not be the sole judges of when to utilize constitutionally sensitive means in pursuing their tasks. The historical judgment, which the Fourth Amendment accepts, is that unreviewed executive discretion may yield too. readily to pressures to-obtain incriminating evidence and overlook potential invasions of privacy and protected speech.17

It may well be that, in the instant case, the Government’s surveillance of Plamondon’s conversations was a reasonable one which readily would have gained prior judicial approval. But this Court “has never sustained a search upon the sole ground that officers reasonably expected to .find evidence of a particular crime and voluntarily confined their activities, to the least intru-. sive means consistent with that end.” Katz, supra, at 356-35.7. The Fourth Amendment contemplates a prior judicial judgment,18 not the risk that executive discretion may be reasonably exercised. This judicial role accords with our basic constitutional doctrine that individual freedoms will best be preserved through . a separation of powers and division of functions among the different branches and levels of Government. Harlan, Thoughts at a Dedication: Keeping the Judicial Function in Balance, 49 A. B. A. J. 943-944 (1963). The independent check upon executive discretion.is not *318satisfied, as the Government' argues, by “extremely limited” post-surveillance judicial review.19 Indeed, post-surveillance review would never reach the- surveillances which failed to result in prosecutions. Prior review by a neutral and detached' magistrate is the time-tested means of effectuating Fourth Amendment rights. Beck v. Ohio, 379 U. S. 89, 96 (1964).

It is true that there have been some exceptions to the warrant requirement. Chimel v. California, 395 U. S. 752 (1969); Terry v. Ohio, 392 U. S. 1 (1968); McDonald v. United States, 335 U. S. 451 (1948); Carroll v. United States, 267 U. S. 132 (1925). But those exceptions are few in number and carefully delineated, Katz, supra, at 357; in general, they serve the legitimate needs of law enforcement officers to protect their own well-being and preserve evidence from destruction. Even while carving out those exceptions, the Court has reaffirmed the principle that the “police must, whenever practicable, obtain advance judicial approval of searches and seizures through the warrant procedure,” Terry v. Ohio, supra, at 20; Chimel v. California, supra, at 762.

The Government argues that the special circumstances applicable to domestic, security, surveillances necessitate a further exception to the warrant requirement. It is urged that the requirement of prior judicial review would obstruct the President in the discharge of his constitutional duty to protect domestic, security. We are told further that these surveillances • are. directed- primarily to the collecting and maintaining of intelligence with *319respect to subversive forces/ and are not an attempt to gather evidence for specific criminal prosecutions. It is said that this type of surveillance should not be subject to traditional warrant requirements which were established to govern investigation' of criminal activity, not ongoing intelligence gathering. Brief for United States 15-16, 23-24; Reply Brief for United States 2-3.

The Government further insists that courts “as a practical matter would have neither-the knowledge nor the techniques necessary to determine whether there was probable cause to believe that surveillance was necessary to protect national security.”. These security problems, the Government contends, involve “a large number of complex and subtle factors” beyond the competence of courts to evaluate. Reply Brief for United States 4.

As a final reason for exemption from a warrant requirement, the Government believes that disclosure to a magistrate of all or even a significant portion of the information' involved in domestic security surveillances “would create serious potential dangers to -the national security and to the lives of informants and agents., . . .' Secrecy is the essential ingredient in intelligence gathering; requiring prior judicial authorization would create a greater 'danger of leaks . . . , because in addition to the judge, you have the clerk,-.the stenographer and some other officer like a law assistant or bailiff who may be apprised of the nature’ of the surveillance-.” Brief for United States 24-25:

These contentions in behalf .of a complete exemption from the warrant requirement, when urged on behalf of the President and the national security in -its domestic implications', merit the most careful consideration. We •certainly do not reject them lightly, especially at a time of worldwide ferment and when civil disorders in this country are more prevalent than iñ the less turbulent *320periods of our history.' There is, no doubt, pragmatic force to the Government’s position.

But we do not think a case has been made for the requested departure from Fourth Amendment standards. The circumstances, described do not justify complete exemption of domestic security surveillance from prior judicial scrutiny. Official surveillance, whether its purpose be criminal investigation or ongoing intelligence gathering, risks infringement of constitutionally protected privacy of speech. Security surveillances are especially sensitive because of the inherent vagueness of the domestic security concept, the necessarily broad and continuing nature of intelligence gathering, and the temptation to utilize such surveillances to oversee political dissent. We recognize, as we have before, the constitutional basis, of the President’s domestic security role, but we think it must be exercised in a manner compatible with the Fourth Amendment. In this case w;e hold that this requires an appropriate prior warrant procedure.

We cannot accept the Government’s argument that internal security matters are too. subtle and complex for judicial evaluation. Courts regularly deal with the most difficult issues of our society. There is no reason to believe that federal judges will be insensitive to or uncomprehending of the issues involved in domestic security cases. Certainly courts can recognize, that domestic security surveillance involves-different considerations from the surveillance of “ordinary crime.” If the threat is too subtle or complex for our senior law enforcement officers to convey its significance to a court, one may question whether there is probable cause for surveillance.

Nor do we believe prior judicial approval will fracture the secrecy essential to official intelligence gathering. The- investigation 'of criminal activity has long *321involved imparting , sensitive information to judicial officers who have respected the confidentialities involved. Judges may be counted upon to. be especially conscious of security requirements in national security cases. Titie III of the Omnibus Crime Control and Safe Streets Act already has imposed this responsibility on the judiciary in connection with such crimes as espionage, sabotage, and treason, .§§ 2516 (l.)(a) and (c), each of which may involve domestic as well as foreign security threats. Moreover, a warrant application involves no public or adversary proceedings: it is an ex parte request before a. magistrate or judge; Whatever security dangers clerical and secretarial personnel may pose can be minimized by proper administrative measures, possibly to the point of allowing the Government itself to provide the necessary clerical assistance.

Thus, we conclude that the Government’s concerns do not justify departure in this case from the customary Fourth Amendment requirement of judicial approval prior to initiation of a search or surveillance. Although some added burden will be imposed upon the Attorney General, this inconvenience is justified in a free society to protect constitutional values. Nor do we think the Government’s domestic surveillance powers will be impaired to any significant degree. A prior warrant establishes presumptive validity of the surveillance and will minimize the burden of justification in post-surveillance judicial review. By no means of least importance will be the reassurance of the public generally that indiscriminate wiretapping and bugging of law-abiding citizens cannot occur.

IV

We emphasize, before concluding this opinion, the scope of our decision. As stated at the outset, this case involves only the domestic aspects of national security. We have not addressed, and express no opinion *322as to, the issues which may be involved with respect to activities of foreign powers or their agents.20 Nor does our decision rest on thé langi^age of § 2511 (3) or any other section of Title III of the Omnibus Crime Control and Safe Streets Act of 1968. That Act does not attempt to define or delineate the powers of the President to meet domestic threats to the national security.

Moreover, we do not hold that the same type of standards and procedures prescribed by Title III are necessarily applicable to this case. , We recognize that domestic security surveillance may involve different policy and practical considerations from the surveillance of “ordinary crime.” The gathering of security intelligence is often long range and involves the interrelation of various sources and types of information. The exact targets of such surveillance may be more difficult to identify than in surveillance operations against many types of crime specified in Title III. Often, too, the emphasis of domestic intelligence gathering is on the prevention of unlawful activity or the enhancement of the Government’s preparedness for some possible future crisis or emergency. Thus, the focus of domestic surveillance may be less precise than that directed against more conventional types of crime.

Given these potential distinctions between Title III criminal surveillances and those involving the domestic security, Congress may wish to. consider protective standards for the latter , which differ from those already prescribed for specified crimes in Title III. Different standards may be compatible with the Fourth Amend*323ment if they are reasonable both in relation to the legitimate need of Government for intelligence information and the protected rights of our citizens. For the warrant application may vary according to the governmental interest to be enforced and the nature of citizen rights deserving protection. As the 'Court said in Camara v. Municipal Court, 387 U. S. 523, 534-535 (1967):

“In cases in which the Fourth Amendment requires that a warrant to search be. obtained, ‘probable cause’ is the standard by which a particular decision -to search is tested against the constitutional mandate of reasonableness. . . . In determining whether a particular inspection is reasonable — -and thus in determining whether there is probable cause to issue a warrant for that inspection — the need for. the inspection must be weighed in terms of these reasonable goals of code enforcement.”

It may be that’ Congress, for example, would judge that the application and affidavit showing probable cause need. not follow the exact requirements of § 2518 but should allege other circumstances more appropriate to domestic security cases; that the request for prior court •authorization could, in sensitive cases, be made to. any member of a specially designated court (e. g., the District Court for the District of Columbia or the Court of Appeals for the District of Columbia Circuit); and that the time and reporting requirements need not be so strict as those in § 2518. ■

The above paragraph does not, -of course, attempt to guide the congressional judgment but rather to delineate the present scope of our own opinion. We do not attempt to detail the precise standards for domestic secu-. rity warrants any more than our decision in Katz sought to set the refined requirements for the specified criminal surveillances which now constitute Title III. We do *324hold, however, that prior judicial approval is required for the type .of domestic security surveillance involved in this case and that such approval may be made in accordance with such reasonable standards as the Congress may prescribe.

V

As the surveillance of Plamondon’s conversations was unlawful, because conducted without prior judicial approval, the courts below correctly held that Alderman v. United States, 394 U. S. 165 (1969), is controlling and that it requires disclosure to the accused of his own im-permissibly intercepted conversations. As stated in Alderman, “the trial court .can and should, where appropriate, place a defendant and his counsel under enforceable orders against unwarranted disclosure of thé materials which they may be entitled to inspect.” 394 U. S., at 185.21

The judgment of the Court of Appeals is hereby

Affirmed.

concurring.

While I join in the opinion of the Court, I add these words in support of it.

This is an important phase in the campaign of the police and intelligence agencies to obtain exemptions from the Warrant Clause of the Fourth Amendment. For, due to the clandestine nature of electronic eavesdropping, the need is acute for placing on the Govern*325ment the heavy burden to show that “exigencies of the situation [make its] course imperative.” 1 Other abuses, such as the search incident to arrest, have been partly deterred by the threat of damage actions against offending officers,2 the risk of adverse publicity, or the possibility of . reform through the political process. These latter safeguards, however, áre ineffective against lawless wiretapping and “bugging” of which their victims are totally unaware. Moreover, ■ even the risk of exclusion of- tainted evidence would here appear to be of negligible deterrent value inasmuch as the United States frankly concedes' that the primary purpose of these searches is to fortify its intelligence collage rather than to accumulate evidence to support indictments and convictions. If the Warrant Clause were held inapplicable here, then the federal intelligence machine would literally enjoy unchecked discretion.

Here, federal agents wish to rummage for months on end through every conversation, no matter how intimate or personal, carried over selected telephone lines, simply to seize those few utterances which may add to their sense of the pulse of a domestic underground.

We are told that one national security wiretap lasted for 14 months and monitored over 900 conversations. Senator -Edward Kennedy found recently that “war-rantless devices accounted for an average of 78 to 209 days of listening per device, as compared with a 13-day per device average for those devices installed under court order.”3 He concluded that the Government’s *326revelations posed “the frightening possibility that the conversations of untold thousands of citizens of this country are being monitored on secret devices which no judge has authorized and which may remain in operation for months and perhaps years at a time.” 4 Even the most innocent and random caller who uses or telephones^ into a tapped line can become a flagged number in the Government’s data bank. See Laird v. Tatum, 1971 Term, No. 71-288.

Such gross invasions of privacy epitomize the very evil to which the Warrant Clause was directed. This' Court .has been the unfortunate witness to the hazards of police intrusions which did not receive prior sanction by independent magistrates. For example, in Weeks v. United States, 232 U. S. 383; Mapp v. Ohio, 367 U. S. 643; and Chimel v. California, 395 U. S. 752, entire homes were ransacked pursuant to warrantless searches.. Indeed, in Kremen v. United States, 353 U. S. 346, the entire contents óf a cabin, totaling more than 890 items (such as “1 Dish Rag”) 5. were seized incident to an arrest of its occupant' and were taken to San Francisco for study. by FBI agents. ' In a similar case, Von Cleef v. New *327Jersey, 395 U. S. 814, police, without a warrant, searched an arrestee’s house for three hours, eventually seizing “several thousand' articles; including books, magazines, catalogues, mailing lists, private correspondence (both open and unopened), photographs, drawings, and film.” Id., at 815. In Silverthorne Lumber Co. v. United.States, 251 U. S. 385, federal agents “without a shadow of authority” raided the offices of one of the petitioners (the proprietors of which had earlier been jailed) and “made a clean sweep of all the books, papers and documents found there.” Justice Holmes, for the Court, termed this tactic an “outrage.” Id., at 390, 391. In Stanford v. Texas, 379 U. S. 476, state police seized more than 2,000 items of literature, including the writings of Mr. Justice Black, pursuant to a general search warrant issued to inspect an alleged subversive’s home.

That “domestic security” is said to be involved here does not draw this case outside the mainstream of Fourth Amendmént law. Rather, the recurring desire of reigning officials to employ dragnet techniques to intimidate their critics lies at the core of that prohibition. For it was such excesses as the use of general warrants and the writs of assistance that led to the ratification of the Fourth Amendment. ' In Entick v. Carrington, 19 How. St. Tr. 1029, 95 Eng. Rep. 807, decided in 1765, one finds a striking parallel to the executive warrants utilized here. The Secretary of State had issued general executive warrants to his messengers authorizing them to roam about and to seize libelous material and libellants of the sovereign. Entick, a critic of the Crown, was the victim of one such general search during which his seditious publications were impounded. He brought a successful damage action for trespass against the messengers. The verdict was sustained on appeal. Lord Camden wrote that if such sweeping tactics were validated, then “the secret cabinets and bureaus of every *328subject in this kingdom will be thrown open to the search and inspection of a messenger, whenever the secretary of state shall think fit to charge, or even to suspect, a person to be the author, printer, or publisher of a seditious libel.” Id., at 1063. In a related and similar proceeding, Huckle v. Money, 2.Wils. K. B. 206, 207, 95 Eng. Rep. 768, 769 (1763), the same judge who presided over Entick’s appeal held for another victim of the same despotic practice, saying “[t]o enter a man’s, house by virtue of a nameless warrant, in order to procure evidence, is worse than the Spanish Inquisition ....” See also Wilkes v. Wood, 19 How. St. Tr. 1153, 98 Eng. Rep. 489 (1763). As early as Boyd v. United States, 116 U. S. 616, 626, and as recently as Stanford v. Texas, supra, at 485-486; Berger v. New York, 388 U. S. 41, 49-50; and Coolidge v. New Hampshire, supra, at 455 n. 9, the tyrannical invasions described and assailed in Entick, Huckle, and Wilkes, practices which also were endured by the colonists,'6 have been rec*329ognized ' as the primary abuses which ensured the Warrant Clausé a prominent place in our Bill of Rights. See J. Landynski, Search and Seizure and the Supreme Court '28-48 (1966). N. Lasson, The History and Development of the .Fourth Amendment to the United States Constitution 43-78 (1937); Note, Warrantless Searches-In Light of Chimel:. A Return To-The Original Understanding, 11 Ariz. L. Rev. 457, 460-476 (1969).

As illustrated by a flood of cases before us this Term, e. g., Laird v. Tatum, No. 71-288; Gelbard v. United States, No. 71-110; United States v. Egan, No. 71-263; United States v. Caldwell, No. 70-57; United States v. Gravel, No. 71-1026; Kleindienst v. Mandel, No. 71-16;. we are currently in the, throes of another national seizure of paranoia, resembling the hysteria which surrounded The Alien and Sedition. Acts, the Palmer Raids, and the McCarthy era. Those who register dissent or who petition their governments for redress are subjected to scrutiny by grand juries,7 by the FBI,8 or even by the military.9 Their associates are in*330terrogated. Their homes are bugged and their telephones are wiretapped. They are befriended by secret government informers.10 Their patriotism and loyalty are ques*331tioned.11 Senator Sam Ervin, who has chaired hearings on military surveillance of civilian dissidents, warns that “it is not an exaggeration to talk in terms of hundreds of thousands of . . . dossiers.” 12 Senator Kennedy, as méntioned supra, found “the frightening possibility that the conversations of untold thousands are being monitored on secret devices.” More than our privacy. is implicated. Also, at stake is the reach of the Government’s power to intimidate its critics.

When the Executive attempts to excuse these tactics as essential to its defense against internál subversion, we are obliged' to remind it, without apology, of this Court’s long commitment to the preservation of the Bill of. Eights from the corrosive environment of precisely such expedi*332ents.13 As Justice Brandéis said, concurring in Whitney v. California, 274 U. S. 357, 377: “Those who won our independence by revo’.ition were not cowards. They did not fear political change. They did not exalt order at the cost of liberty.” Chief Justice Warren put it this way in United States v. Bobel, 389 U. S. 258, 264: “[T]his concept of ‘national defense’ cannot be deemed an end in itself, justifying any .. . power designed to promote such a goal. Implicit in the term ‘national defense’ is the notion of defending those values and ideas which set this Nation apart. . . . It would indeed be ironic if, in the name of national defense, we would sanction the subversion of . . . those liberties . . . which [make] the defense of the Nation worthwhile.”

The Warrant Clause has stood as a barrier against intrusions by officialdom into the privacies of life. But if that barrier were lowered now to permit suspected subversives’ most intimate conversations to be pillaged then why could not their abodes or mail be secretly searched by the same authority? To defeat so'terrifying a claim of inherent power we need only stand by the enduring values served by the Fourth Amendment. As we stated last Term in Coolidge v. New Hampshire, 403 U. S. 443, 455: “In times of unrest, whether caused by crime or racial conflict or fear of internal subversion, -this basic law *333and the values that.it represents may appear unrealistic or ‘extravagant’ to some. But the values were those of the authors of our fundamental constitutional concepts. In times not altogether unlike 'our own they won ... a right of personal security against arbitrary intrusions .... If times have changed, reducing everyman’s scope , to do as . he pleases in an urban and industrial world,- the changes, have made the values served.by the Fourth Amendment more, not less, important.” We have as much, or more to fear from the erosion of our sense of privacy and independence by the omnipresent electronic ear of the Government as we do from the likelihood that fomenters of domestic upheaval will modify our form of governing.14

*334APPENDIX TO OPINION OF DOUGLAS, J., CONCURRING

FEDERAL WIRETAPPING AND BUGGING 1969-1970

Days in Number' Use Year Days in Use Minimum Maximum (Rounded) (Rounded) z a &■ a

1969 30 462 94 8,100 20,800

1970 180 2,363 113 8,100 22,600

Average Days in Use Per Device Court Executive Ordered Ordered Devices Ratio of Days Used Executive Ordered: Court Ordered

Minimum Devices Minimum Maximum 3 H •R h si r*

17.5* 3.4 15.4 86.2 221.3 13.1 71.7 200.0 * O CO W5 03 030 03 03 t — i i — (

concurring in the judgment.

This case arises out of a two-count indictment charging conspiracy to injure and injury to Government property. Count I charged Robert Plamondon and two codefend-ants with conspiring with a fourth person to injure Government property with dynamite. Count II charged Plamondon alone with dynamiting and injuring Government property in Ann Arbor, Michigan. The defendants moved to compel the United States to disclose, among other,things, any logs and records of electronic-surveillance directed at them,'at unindicted coconspirators, or at any premises of the defendants or coconspirators. They also moved for a hearing to determine, whether any electronic surveillance disclosed had tainted the evidence On which the grand jury indictment was based and which the Government intended to use at trial. They asked for dismissal of the indictment if such taint were determined to exist. Opposing the motion, the United States submitted an affidavit of the Attorney General of the United States disclosing that “[t]he defendant Pla-mondon has participated, in conversations which were overheard by Government agents who were monitoring wiretaps which were being employed to gather intelligence information deemed necessary to protect the nation from attempts of domestic organizations to attack and subvert the existing structure of the Government,”the wiretaps having been expressly approved by the Attorney General. The records of the intercepted conversations and copies of the memorandum reflecting the Attorney General’s approval were submitted under seal and solely for the Court’s in camera inspection.1

*336As characterized by the District Court, the position of the United States was that the electronic monitoring of Plamondon’s conversations without judicial warrant was a lawful. exercise of the power of the President to safeguard the national security. The District Court granted the motion of defendants, holding that the President had no constitutional power- to employ electronic surveillance without warrant to gather information about domestic organizations. Absent probable cause and judicial authorization, the challenged wiretap infringed Plamondon’s Fourth Amendment rights. The court ordered the Government to disclose to defendants the records of the .monitored conversations and directed that a hearing be held to determine the existence of taint either in the indictment or in the evidence to be introduced at trial.

The Government’s petition for mandamus to require the District Court to vacate its order was denied by the Court of Appeals. 444 F. 2d 651 (CA6 1971). That court held that the Fourth Amendment barred warrantless electronic surveillance of domestic organizations even if at the direction of the President. It agreed with the District Court that because the wiretaps involved were therefore constitutionally infirm, the United States must turn over to defendants the records of overheard conversations- for the purpose of determining whether the Government’s evidence was tainted.

I would affirm the Court of Appeals but on the statutory ground urged by defendant-respondents (Brief 115) without reaching or intimating any views with respect *337to the constitutional issue decided by both the District Court and the Court of Appeals.

Title III of the Omnibus Crime Control and Safe Streets Act of 1968, 18 U. S. C. §§ 2610-2520, forbids, under pain of criminal penalties and civil actions for damages, any wiretapping or eavesdropping not undertaken in accordance with specified procedures for obtaining judicial warrants authorizing the surveillance. Section 2511 (1) establishes a general prohibition against electronic eavesdropping “[ejxcept as otherwise specifically provided” in the statute. Later sections provide detailed procedures for judicial authorization of official interceptions of oral communications; when these procedures are followed the interception is not subject to the prohibitions of §2511 (1). Section 2511 (2), how-evér, specifies other situations in which the general prohibitions of § 2511 (1). do not apply. In addition, § 2511 (3) provides that:

“Nothing contained in this chapter or in section .605 of the Communications Act of 1934 (48 Stat. 1143; 47 U. S. C.. 605) shall limit the constitutional power of,the President to take such measures as he deems necessary to protect the Nation against actual or potential attack or other hostile acts of a foreign power, to obtain foreign intelligence information deemed essential to the security of the United States, or to protect national security information against foreign intelligence activities. Nor shall anything contained in this chapter be deemed to 'limit the constitutional power of the President to take such measures as he deems necessary to protect the United States against the overthrow of the Government by force or other unlawful means, or against any other clear and present danger to the structure or existence of the Government. The con*338tents of any wire or oral communication intercepted by authority of the President in the exercise of the foregoing powers may be received in evidence in any trial hearing, or other proceeding only where such interception was reasonable, and shall not be otherwise used or. disclosed except as is necessary to. implement that power.”

It is this subsection that lies at the heart of this case.

The interception here was without judicial warrant, it was hot covered by the provisions, of § 2511 (2) and it is too clear for argument that it- is illegal under § 2511 (1) unless it is saved by §2511 (3). The majority asserts that § 2511 (3) is a “disclaimer” but not an “exception.” But however,it is labeled, it is apparent from the face of the section and its legislative history that if this interception is one of those described in § 2511 (3j, it is not reached by the statutory ban bn unwarranted electronic eavesdropping.2

The. defendants in the District Court moved for the production of the logs, of any electronic surveillance to which they might have been subjected. "The Govern-*339melit responded that conversations of Plamondon had been intercepted but took the position that turnover of surveillance records was not necessary because the interception complied with the law. Clearly, for the Government to prevail it was necessary to demonstrate, first, that the interception involved was not subject to the statutory requirement of judicial approval for wiretapping because the surveillance was within the scope of § 2511 (3); and, secondly, if the -Act did not forbid the warrantless wiretap, that the-surveillance was consistent with the Fourth Amendment.

The United States has made no claim in this case that the statute may not constitutionally be applied to the surveillance at issue here.3 Nor has it denied that to *340comply with the Act the surveillance must either be supported by a warrant pr fall within the bounds of the exceptions provided by § 2511 (3). Nevertheless, as I read the opinions of-the District Court and the Court of Appeals, neither court stopped to., inquire whether the chair lenged interception was illegal under the statute but proceeded directly to the constitutional issue without adverting to the time-honored rule that courts should abjure constitutional issues except where necessary to decision of the case before them. Ashwander v. Tennessee Valley Authority, 297 U. S. 288, 346-348 (1936) (concurring opinion). Because I conclude that on the record before us the surveillance undertaken by the Government in this case was illegal under the statute itself, I find it unnecessary, and therefore improper, to consider or decide the constitutional questions which the courts below improvidently reached.

The threshold statutory question is simply put: Was the electronic surveillance undertaken by the Government in this case a measure- deemed necessary by the President to implement either the first or second branch of the exception carved out by § 2511 (3) to the general requirement of a warrant? .

The answer, it seems to me, must turn on the affidavit of the Attorney General offered by the United States in opposition to defendants’ motion to disclose surveillance records. It is apparent that there is nothing whatsoever in this affidavit suggesting that the surveillance was *341undertaken within the first branch of the § 2511 (3) exception, that is,- to protect against foreign attack, to gather foreign intelligence or to protect national security information. The sole assertion was that the monitoring at issue was employed to gather intelligence information “deemed necessary to protect the nation from attempts of domestic organizations to attack and subvert the existing structure of the Government.” App. 20.

Neither can I-conclude from this characterization that the wiretap employed here fell within the exception recognized by the second sentence of § 2511 (3); for it utterly fails to assume responsibility for the judgment that Congress demanded: that the surveillance was necessary to prevent overthrow by force or other unlawful means or that there was any other clear and present dangér to the structure or existence of the Government. The affidavit speaks.only of attempts to attack or subvert; it makes no reference to force or unlawfulness; it articulates no conclusion that the attempts involved any clear and present danger .to the existence or structure of the Government.

The shortcomings of the affidavit when. measured against § 2511 (3) are patent. Indeed, the United States in oral argument conceded no less. The specific inquiry put to Government counsel was: “Do you think the' affidavit, standing alone, satisfies the Safe Streets Act?” The Assistant Atterney General answered “No, sir. We do not rely upon the affidavit itself . . . .” Tr. of Oral -Arg. 15.4

Government ciounsel, however, seek to save their case by reference to the in camera exhibit submitted to the *342District Court to supplement - the Attorney General's affidavit.5- It is said that the exhibit includes the request for wiretap approval submitted to the Attorney General, that the request asserted the need to avert a clear and present danger to the structure and existence of the Government, and that the Attorney General endorsed his approval on the request.6 But' I am unconvinced that the mere endorsement of the Attorney General on the. request for approval submitted to him must be taken as the Attorney General’s own opinion that the wiretap was necessary to . avert a clear and present danger to the existence or structure of the Government *343when, in an affidavit later filed in court specifically characterizing the purposes of the interception and at least impliedly the grounds for his prior approval, the Attorney General said only that the tap was undertaken to secure intelligence thought necessary to protect against attempts to attack.and subvert the structure,of Government. If the Attorney General’s approval of the interception is to be given a judicially cognizable meaning different from the meaning he seems to have ascribed to it in his affidavit filed in court, there obviously must be further proceedings in the District Court.

Moreover, I am reluctant to proceed in the first instance to examine the in camera material and either sustain ór reject the. surveillance ■ as a necessary measure to avert the dangers referred to in. §2511(3). What Congress excepted from the warrant requirement was a surveillance which the President would assume responsibility for deeming an essential measure to protect against clear and present danger.. No judge can ■satisfy this congressional requirement.

Without the necessary threshold determination, the interception is, in my opinion, contrary to the terms of the statute'and subject therefore to the prohibition contained in § 2515 against the use of the fruits of the warrantless electronic surveillance as evidence at any trial.7

There remain two additional interrelated reasons for, net reaching the constitutional issue. First, even if it were determined that the Attorney General purported to *344authorize an electronic surveillance for purposes exempt irom the general provisions of the Act, there would remain the issue whether his discretion was properly authorized. The United States concedes that the act of the Attorney General authorizing a warrantless wiretap is subject to judicial review to some extent, Brief for United States 21-23, and it seems improvident to proceed to constitutional questions until it x& determined that the Act itself does not bar the interception here in question.

Second, and again on the assumption that the surveillance here involved-fell within the exception providec by §2511 (3), no constitutional issue need be reached in this case if the fruits of the wiretap were inadmissible on statutory grounds in the criminal proceedings pending against respondent Plamondon. Section 2511 (3) itself states that “[t]he contents of any wire or oral communication intercepted by authority of the President in the exercise of the foregoing powers may be received in evidence in any trial hearing, or other proceeding only where such interception was reasonable, and shall not be otherwise used or disclosed except as is necessary to implement that power.” (Emphasis added.) There has been no determination by the District Court that it would be reasonable to use the fruits of the wiretap against Plamondon or that it would be necessary to do so to implement the purposes for which the tap was authorized.

My own conclusion, again, is that, as long as non-constitutional, statutory grounds for excluding the evidence or its fruits have not been disposed of, it is improvident to reach the constitutional issue.

I would thus affirm the -judgment of the Court of Appeals unless the Court is prepared to reconsider the necessity for - an adversary, rather than an in camera, hearing with respect to taint. If in camera proceedings are sufficient and no taint is discerned by the judge, this case is over, whatever the legality of the tap.