23 Opinion of CLARK, J. searches and seizures." 367 U. S., at 655. We now face the specific question as to whether Mapp requires the exclusion of evidence in this case which the California District Court of Appeal has held to be lawfully seized. It is perhaps ironic that the initial test under the Mapp holding comes from California, whose decision voluntarily to adopt the exclusionary rule in 1955 has been commended by us previously. See Mapp v. Ohio, supra, at 651–652; Elkins v. United States, 364 U. S. 206, 220 (1960). Preliminary to our examination of the search and seizures involved here, it might be helpful for us to indicate what was not decided in Mapp. First, it must be recognized that the "principles governing the admissibility of evidence in federal criminal trials have not been restricted . . . to those derived solely from the Constitution. In the exercise of its supervisory authority over the administration of criminal justice in the federal courts... this Court has . . . formulated rules of evidence to be applied in federal criminal prosecutions." McNabb v. United States, 318 U. S. 332, 341 (1943); cf. Miller v. United States, 357 U. S. 301 (1958); Nardone v. United States, 302 U. S. 379 (1937). Mapp, however, established no assumption by this Court of supervisory authority over state courts, cf. Cleary v. Bolger, 371 U. S. 392, 401 (1963), and, consequently, it implied no total obliteration of state laws relating to arrests and searches in favor of federal law. Mapp sounded no death knell for our federalisın; rather, it echoed the sentiment of Elkins v. United States, supra, at 221, that "a healthy federalism depends upon the avoidance of needless conflict between state and federal courts" by itself urging that "[f]ederal-state cooperation in the solution of crime under constitutional standards will be promoted, if only by recognition of their now mutual obligation to respect the same fundamental criteria in their approaches." 367 U. S., at 658. (Emphasis added.) Second, Mapp did not attempt the impossible task of lay 699-272 O-63–6 Opinion of CLARK, J. 374 U.S. ing down a "fixed formula" for the application in specific cases of the constitutional prohibition against unreasonable searches and seizures; it recognized that we would be "met with 'recurring questions of the reasonableness of searches'" and that, "at any rate, '[r]easonableness is in the first instance for the [trial court] ... to determine,' id., at 653, thus indicating that the usual weight be given to findings of trial courts. Mapp, of course, did not lend itself to a detailed explication of standards, since the search involved there was clearly unreasonable and bore no stamp of legality even from the Ohio Supreme Court. Id., at 643-645. This is true also of Elkins v. United States, where all of the courts assumed the unreasonableness of the search in question and this Court "invoked" its "supervisory power over the administration of criminal justice in the federal courts," 364 U. S., at 216, in declaring that the evidence so seized by state officers was inadmissible in a federal prosecution. The prosecution being in a federal court, this Court of course announced that "[t]he test is one of federal law, neither enlarged by what one state court may have countenanced, nor diminished by what another may have colorably suppressed." Id., at 224. Significant in the Elkins holding is the statement, apposite here, that "it can fairly be said that in applying the Fourth Amendment this Court has seldom shown itself unaware of the practical demands of effective criminal investigation and law enforcement." Id., at 222. Implicit in the Fourth Amendment's protection from unreasonable searches and seizures is its recognition of individual freedom. That safeguard has been declared to be "as of the very essence of constitutional liberty" the guaranty of which "is as important and as imperative as are the guaranties of the other fundamental rights of the individual citizen...." Gouled v. United States, 255 U. S. 298, 304 (1921); cf. Powell v. Alabama, 287 U. S. 23 Opinion of CLARK, J. 45, 65-68 (1932). While the language of the Amendment is "general," it "forbids every search that is unreasonable; it protects all, those suspected or known to be offenders as well as the innocent, and unquestionably extends to the premises where the search was made. Go-Bart Importing Co. v. United States, 282 U. S. 344, 357 (1931). Mr. Justice Butler there stated for the Court that "[t]he Amendment is to be liberally construed and all owe the duty of vigilance for its effective enforcement lest there shall be impairment of the rights for the protection of which it was adopted." Ibid. He also recognized that "[t]here is no formula for the determination of reasonableness. Each case is to be decided on its own facts and circumstances." Ibid.; see United States v. Rabinowitz, 339 U. S. 56, 63 (1950); Rios v. United States, 364 U. S. 253, 255 (1960). This Court's long-established recognition that standards of reasonableness under the Fourth Amendment are not susceptible of Procrustean application is carried forward when that Amendment's proscriptions are enforced against the States through the Fourteenth Amendment. And, although the standard of reasonableness is the same under the Fourth and Fourteenth Amendments, the demands of our federal system compel us to distinguish between evidence held inadmissible because of our supervisory powers over federal courts and that held inadmissible because prohibited by the United States Constitution. We reiterate that the reasonableness of a search is in the first instance a substantive determination to be made by the trial court from the facts and circumstances of the case and in the light of the "fundamental criteria" laid down by the Fourth Amendment and in opinions of this Court applying that Amendment. Findings of reasonableness, of course, are respected only insofar as consistent with federal constitutional guarantees. As we have stated above and in other cases in Opinion of CLARK, J. 374 U.S. volving federal constitutional rights, findings of state courts are by no means insulated against examination here. See, e. g., Spano v. New York, 360 U. S. 315, 316 (1959); Thomas v. Arizona, 356 U. S. 390, 393 (1958); Pierre v. Louisiana, 306 U. S. 354, 358 (1939). While this Court does not sit as in nisi prius to appraise contradictory factual questions, it will, where necessary to the determination of constitutional rights, make an independent examination of the facts, the findings, and the record so that it can determine for itself whether in the decision as to reasonableness the fundamental-i. e., constitutional criteria established by this Court have been respected. The States are not thereby precluded from developing workable rules governing arrests, searches and seizures to meet "the practical demands of effective criminal investigation and law enforcement" in the States, provided that those rules do not violate the constitutional proscription of unreasonable searches and seizures and the concomitant command that evidence so seized is inadmissible against one who has standing to complain. See Jones v. United States, 362 U. S. 257 (1960). Such a standard implies no derogation of uniformity in applying federal constitutional guarantees but is only a recognition that conditions and circumstances vary just as do investigative and enforcement techniques. Applying this federal constitutional standard we proceed to examine the entire record including the findings of California's courts to determine whether the evidence seized from petitioners was constitutionally admissible under the circumstances of this case. II. The evidence at issue, in order to be admissible, must be the product of a search incident to a lawful arrest, since the officers had no search warrant. The lawfulness of the arrest without warrant, in turn, must be based upon 23 Opinion of Clark, J. probable cause, which exists "where "the facts and circumstances within their [the officers'] knowledge and of which they had reasonably trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that' an offense has been or is being committed." Brinegar v. United States, 338 U. S. 160, 175-176 (1949), quoting from Carroll v. United States, 267 U. S. 132, 162 (1925); accord, People v. Fischer, 49 Cal. 2d 442, 317 P. 2d 967 (1957); Bompensiero v. Superior Court, 44 Cal. 2d 178, 281 P. 2d 250 (1955). The information within the knowledge of the officers at the time they arrived at the Kers' apartment, as California's courts specifically found, clearly furnished grounds for a reasonable belief that petitioner George Ker had committed and was committing the offense of possession of marijuana. Officers Markman and Warthen observed a rendezvous between Murphy and Ker on the evening of the arrest which was a virtual reenactment of the previous night's encounter between Murphy, Terrhagen and Sergeant Cook, which concluded in the sale by Murphy to Terrhagen and the Sergeant of a package of marijuana of which the latter had paid Terrhagen for one pound which he received from Terrhagen after the encounter with Murphy. To be sure, the distance and lack of light prevented the officers from seeing and they did not see any substance pass between the two men, but the virtual identity of the surrounding circumstances warranted a strong suspicion that the one remaining element—a sale of narcotics-was a part of this encounter as it was the previous night. But Ker's arrest does not depend upon this single episode with Murphy. When Ker's U-turn thwarted the officer's pursuit, they learned his name and address from the Department of Motor Vehicles and reported the occurrence to Officer Ber Berman, in turn, revealed information from an informer whose reliability had been tested previously, as |