Knock-and-announce rule: an overview
Under the common law knock-and-announce rule, a police officer executing a search warrant generally must not immediately force his or her way into a residence. Instead, he or she must first knock, identify himself or herself and his or her intent, and wait a reasonable amount of time for the occupants to let him or her into the residence. The Supreme Court has held that the knock-and-announce rule forms part of a judge's inquiry into the reasonableness of a search under the Fourth Amendment. See Wilson v. Arkansas, 514 U.S. 927 (1995).
The Supreme Court identified several reasons supporting the rule in Hudson v. Michigan, 547 U.S. 586 (2006). These include preventing accidental injuries to officers and occupants, limiting property damage, and protecting occupants' privacy and dignity. This rule, however, does not protect occupants from government seizure of their property. Accordingly, although the exclusionary rule may apply to some police violations of the rule, it does not apply to all.
A police officer is not required to knock and announce if doing so would be unreasonable, e.g. if there is a risk of injury to the police officer executing the search warrant or a risk of the occupants destroying the sought-after evidence between the police officer's knock and his or her entry. The applicability of this exception is determined on a case-by-case basis. For example, in Richards v. Wisconsin, 520 U.S. 358 (1997), the Supreme Court held that there is no blanket exception to the knock-and-announce rule for searches in felony drug cases. Officers may, however, seek a "no-knock" warrant in advance if they suspect that a no-knock entry will be justified when they serve the warrant.
In practice, over the past decade, police officers have increasingly relied on no-knock warrants, particularly in drug cases and especially in major cities. There has been a corresponding increase in the number of innocent persons accidentally injured or killed by police officers executing no-knock warrants.
Illustrative caselaw
See, e.g. Hudson v. Michigan, 547 U.S. 586 (2006).
https://www.law.cornell.edu/wex/knock-and-announce_rule