Opinion of O'CONNOR, J.

less expected, that the Compact might result in a monetary award of prejudgment interest over half a century after its signing.

The Court ignores all of this in awarding prejudgment interest to Kansas, seizing instead upon the compensatory rationale behind the criticism of the common law rule and awards of prejudgment interest on unliquidated claims for damages in general. See ante, at 10-11. I do not dispute that awards of interest are compensatory in nature or that, as a general matter, "a monetary award does not fully compensate for an injury unless it includes an interest component." Ante, at 10; see also National Gypsum Co., 515 U. S., at 195, n. 7. But, as the Court itself recognizes, see ante, at 11, our precedents make clear that, at least today and in the absence of a governing statute, awards of prejudgment interest on unliquidated claims for damages are governed not by any "rigid theory of compensation for money withheld," but rather by "considerations of fairness." Blau, 368 U. S., at 414 (internal quotation marks and citation omitted); see, e. g., General Motors Corp. v. Devex Corp., 461 U. S. 648, 651-653, and n. 5 (1983); Funkhouser, 290 U. S., at 168169. This is especially so where, as here, we are dealing with suits by one governmental body against another. See West Virginia v. United States, 479 U. S. 305, 309–312 (1987); Board of Comm'rs of Jackson Cty., 308 U. S., at 349–353.

There is nothing fair about awarding prejudgment interest as a remedy for the Compact's breach when all available evidence suggests that the signatories to the Compact neither intended nor contemplated such an unconventional remedy. Many compacts between States are old; suits involving compacts concerning water rights are late in starting and are invariably long pending; and, because statutes of limitation or the doctrine of laches is rarely available to preclude the steady buildup of prejudgment interest, the amount of such interest can become quite large, as Kansas' claim for approximately $41 million illustrates. See ante, at 9, n. 2. One

Opinion of O'CONNOR, J.

would think that, particularly in such circumstances, even the most rudimentary conception of fairness would dictate that the Court ought not to interpret a contract between two States as exposing one of them to liability under a novel legal principle some 50 years later without some indication that the States might have contemplated such exposure in conjunction with the contractual rights and duties expressed in their compact. Contrary to the Court's apparent belief, see ante, at 11-12, n. 4, nothing about such a contextualized historical approach would create an across-the-board incentive for the continued breach of interstate compacts entered into before 1987, especially given the prospect of large and uncertain damages awards. Had Kansas and Colorado anticipated or even suspected what the Court today effects, they almost certainly would have negotiated a provision in the Compact to address the situation. States in the future very likely will do so in the wake of the Court's decision, which creates a very different backdrop from the one against which Kansas and Colorado operated. In the absence of such a provision, however, "the loss [as to interest] should remain where it has fallen." Board of Comm'rs of Jackson Cty., supra, at 353; see Third Report § XI, at 101 ("Prejudgment interest here neither takes from those who benefitted, nor goes to those who were injured").

For the foregoing reasons, I respectfully dissent from the Court's award of prejudgment interest.

Syllabus

KYLLO v. UNITED STATES

CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR THE NINTH CIRCUIT

No. 99-8508. Argued February 20, 2001-Decided June 11, 2001 Suspicious that marijuana was being grown in petitioner Kyllo's home in a triplex, agents used a thermal-imaging device to scan the triplex to determine if the amount of heat emanating from it was consistent with the high-intensity lamps typically used for indoor marijuana growth. The scan showed that Kyllo's garage roof and a side wall were relatively hot compared to the rest of his home and substantially warmer than the neighboring units. Based in part on the thermal imaging, a Federal Magistrate Judge issued a warrant to search Kyllo's home, where the agents found marijuana growing. After Kylo was indicted on a federal drug charge, he unsuccessfully moved to suppress the evidence seized from his home and then entered a conditional guilty plea. The Ninth Circuit ultimately affirmed, upholding the thermal imaging on the ground that Kyllo had shown no subjective expectation of privacy because he had made no attempt to conceal the heat escaping from his home. Even if he had, ruled the court, there was no objectively reasonable expectation of privacy because the thermal imager did not expose any intimate details of Kyllo's life, only amorphous hot spots on his home's exterior.

Held: Where, as here, the Government uses a device that is not in general public use, to explore details of a private home that would previously have been unknowable without physical intrusion, the surveillance is a Fourth Amendment "search," and is presumptively unreasonable without a warrant. Pp. 31-41.

(a) The question whether a warrantless search of a home is reasonable and hence constitutional must be answered no in most instances, but the antecedent question whether a Fourth Amendment "search" has occurred is not so simple. This Court has approved warrantless visual surveillance of a home, see California v. Ciraolo, 476 U. S. 207, 213, ruling that visual observation is no "search" at all, see Dow Chemical Co. v. United States, 476 U. S. 227, 234–235, 239. In assessing when a search is not a search, the Court has adapted a principle first enunciated in Katz v. United States, 389 U. S. 347, 361: A "search" does not occur-even when its object is a house explicitly protected by the Fourth Amendment-unless the individual manifested a subjective

Syllabus

expectation of privacy in the searched object, and society is willing to recognize that expectation as reasonable, see, e. g., California v. Ciraolo, supra, at 211. Pp. 31-33.

(b) While it may be difficult to refine the Katz test in some instances, in the case of the search of a home's interior-the prototypical and hence most commonly litigated area of protected privacy-there is a ready criterion, with roots deep in the common law, of the minimal expectation of privacy that exists, and that is acknowledged to be reasonable. To withdraw protection of this minimum expectation would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment. Thus, obtaining by sense-enhancing technology any information regarding the home's interior that could not otherwise have been obtained without physical "intrusion into a constitutionally protected area," Silverman v. United States, 365 U. S. 505, 512, constitutes a search—at least where (as here) the technology in question is not in general public use. This assures preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted. Pp. 33-35.

(c) Based on this criterion, the information obtained by the thermal imager in this case was the product of a search. The Court rejects the Government's argument that the thermal imaging must be upheld because it detected only heat radiating from the home's external surface. Such a mechanical interpretation of the Fourth Amendment was rejected in Katz, where the eavesdropping device in question picked up only sound waves that reached the exterior of the phone booth to which it was attached. Reversing that approach would leave the homeowner at the mercy of advancing technology-including imaging technology that could discern all human activity in the home. Also rejected is the Government's contention that the thermal imaging was constitutional because it did not detect "intimate details." Such an approach would be wrong in principle because, in the sanctity of the home, all details are intimate details. See, e. g., United States v. Karo, 468 U. S. 705; Dow Chemical, supra, at 238, distinguished. It would also be impractical in application, failing to provide a workable accommodation between law enforcement needs and Fourth Amendment interests. See Oliver v. United States, 466 U. S. 170, 181. Pp. 35-40.

(d) Since the imaging in this case was an unlawful search, it will remain for the District Court to determine whether, without the evidence it provided, the search warrant was supported by probable cause-and if not, whether there is any other basis for supporting admission of that evidence. P. 40.

190 F.3d 1041, reversed and remanded.

Opinion of the Court

SCALIA, J., delivered the opinion of the Court, in which SOUTER, THOMAS, GINSBURG, and BREYER, JJ., joined. STEVENS, J., filed a dissenting opinion, in which REHNQUIST, C. J., and O'CONNOR and KENNEDY, JJ., joined, post, p. 41.

Kenneth Lerner, by appointment of the Court, 531 U. S. 955, argued the cause and filed briefs for petitioner.

Deputy Solicitor General Dreeben argued the cause for the United States. With him on the brief were former Solicitor General Waxman, Assistant Attorney General Robinson, Irving L. Gornstein, and Deborah Watson.*

JUSTICE SCALIA delivered the opinion of the Court.

This case presents the question whether the use of a thermal-imaging device aimed at a private home from a public street to detect relative amounts of heat within the home constitutes a "search" within the meaning of the Fourth Amendment.

I

In 1991 Agent William Elliott of the United States Department of the Interior came to suspect that marijuana was being grown in the home belonging to petitioner Danny Kyllo, part of a triplex on Rhododendron Drive in Florence, Oregon. Indoor marijuana growth typically requires highintensity lamps. In order to determine whether an amount of heat was emanating from petitioner's home consistent with the use of such lamps, at 3:20 a.m. on January 16, 1992, Agent Elliott and Dan Haas used an Agema Thermovision 210 thermal imager to scan the triplex. Thermal imagers detect infrared radiation, which virtually all objects emit but which is not visible to the naked eye. The imager converts radiation into images based on relative warmth-black

*Briefs of amici curiae urging reversal were filed for the Liberty Project by Julie M. Carpenter; and for the National Association of Criminal Defense Lawyers et al. by James J. Tomkovicz, Lisa B. Kemler, and Steven R. Shapiro.