A no-knock warrant is a warrant that gives the police and other law enforcement officers legal permission to enter a premises without knocking first to alert the occupants of their presence.
In order for a no-knock warrant to be issued, there must be just cause, and a judge must sign off on it.
No-knock warrants are often issued in circumstances where a judge may fear that evidence could be destroyed or that law enforcement could be in danger if a no-knock warrant is not issued. So, for example, a no-knock warrant can be issued if it is suspected that a violent criminal may be inside the building, or that drugs or other implicating materials may be inside.