When do the police need a warrant to make an arrest?
As long as the police have good reason (called "probable cause") to believe that a crime has been committed and that the person they want to arrest committed the crime, they can, with just one exception, make an arrest without asking a judge for a warrant.
The exception? There are few places where the adage "a man's home is his castle" still applies, and an arrest at home is one of them. The police must have a warrant to arrest a person at home if the arrest is for a nonserious offense -- such as a simple assault -- and there is no fear that the person they want to arrest will destroy evidence or cause harm to the public.
Copyright 2005 Nolo