Knock-and-announce, in United States law of criminal procedure, is an ancient common-law principle which requires law enforcement officers to announce their presence and provide residents with an opportunity to open the door to the residence when conducting a search.

In Wilson v. Arkansas, 514 U.S. 927 (1995), the Supreme Court outlined that a police officer does not have to knock and announce when he or she has a reasonable suspicion that one of the following is present:

• "Circumstances presen[t] a threat of physical violence"
• There is "reason to believe that evidence would likely be destroyed if advance notice were given"
• Knocking and announcing would be "futile" Richards v. Wisconsin, 520 U.S. 385, 394 (1997)

Jerry Seper wrote in the Washington Times:

In 1995, the Supreme Court ruled in an Arkansas case that the Fourth Amendment required police officers executing search warrants to knock and announce their presence before entering. The decision also recognized that in some circumstances a search still would be legal without a knock-and-announce by police. [1]

In Hudson v. Michigan (2006), the Supreme Court ruled that a violation of the knock and announce rule does not require the suppression of evidence using the Exclusionary rule.