In most circumstances, the police do not need to ask a judge for a warrant in order to make an arrest, provided the police have a good reason -- known as "probable cause" -- to believe that 1) a crime has been committed and 2) that the individual that they want to arrest committed the crime. The exception to this is if law enforcement officers want to arrest an individual at his or her home; in this situation, police officers must usually have an arrest warrant in order to do so.