438 U.S. 154 (1978)
Franks v. Delaware is a cornerstone of Fourth Amendment jurisprudence that operationalizes the Warrant Clause's requirement that probable cause be supported by "Oath or affirmation." Before Franks, many jurisdictions categorically barred defendants from probing the truthfulness of a facially sufficient warrant affidavit.
Does the Fourth Amendment require that a defendant be afforded an evidentiary hearing to challenge the veracity of statements in a search-warrant affidavit, and if so, what threshold showing must the defendant make and what is the appropriate remedy?
A warrant affidavit is presumed valid. However, where the defendant makes a substantial preliminary showing that (1) a false statement knowingly and intentionally, or with reckless disregard for the truth, was included by the affiant in the warrant affidavit, and (2) the allegedly false statement is necessary to the magistrate's finding of probable cause, the Fourth Amendment requires that the trial court conduct an evidentiary hearing (a "Franks hearing"). At that hearing, if the defendant proves by a preponderance of the evidence that the affiant made the challenged false statement knowingly and intentionally, or recklessly, then the false material must be set aside; if the remaining content of the affidavit is insufficient to establish probable cause, the warrant must be voided and the fruits of the search excluded. Innocent mistakes or mere negligence do not suffice, and the focus is on the affiant's veracity, not on the informant's, unless the affiant's recklessness relates to the use of the informant's statements.
Yes. The Fourth Amendment requires a hearing when a defendant makes a substantial preliminary showing that the affiant included a knowingly or recklessly false, material statement in the warrant affidavit. If, at the hearing, the defendant proves the allegation by a preponderance and the affidavit—purged of the false material—no longer establishes probable cause, the warrant is invalid and the evidence must be suppressed. The judgment was reversed and the case remanded for proceedings consistent with this standard.
Franks v. Delaware institutionalizes a vital check on the warrant process by allowing defendants, in limited but meaningful circumstances, to test the truthfulness of the facts presented to a magistrate. It clarified both the substantive standard (knowing or reckless falsehoods, not negligence) and the procedural gatekeeping (a substantial preliminary showing backed by an offer of proof) required to obtain a hearing. The case also introduced the "materiality" overlay: suppression is warranted only if the falsehood was necessary to probable cause. For law students, Franks is foundational for understanding suppression practice, the mechanics of pretrial litigation, and the balance the Court strikes between effective policing and the constitutional demand for fair, reliable warrant procedures. In subsequent doctrine, including United States v. Leon, Franks also delineates a recognized exception to the good-faith rule: officers cannot claim good faith where a magistrate was misled by an affiant's knowing or reckless falsehoods.