The knock-and-announce rule stipulates that law enforcement personnel must make their presence known prior to executing a search warrant. Its main purposes are to guard against unnecessary property damage and to reduce the risk of potentially dangerous encounters. As per the Fourth Amendment, it also protects citizens from unreasonable search and seizure.
Historically, the U.S. Supreme Court has ruled that courts can take adherence or lack of adherence to the knock-and-announce rule into account when determining if a search was "reasonable" under the Fourth Amendment.
However, the U.S. Supreme Court has also held that "knock and announce" is not universally mandated. Specifically, it has determined that there may be some circumstances in which law enforcement personnel do not have to follow the rule, and has given lower courts latitude to decide what they are.