If you witness a crime, it's your primary duty to report it to the police. If the crime is a misdemeanor, the police are generally not able to cite the perpetrator for a crime not committed in their presence. So if you witness a misdemeanor, you may make a citizen's arrest or sign a complaint form. In either case, you could be called to testify later in court, although this is by no means always necessary. Most states provide for citizen's arrest powers if a misdemeanor has been committed in the citizen's presence or if the citizen is aware that a felony has been committed. Generally, a police officer can arrest a suspected felon on the grounds of probable cause, and the officer is given immunity from subsequent civil lawsuits if the arrest turns out to have been unjustified. In most states, the citizen's right to arrest does not include any rights to search the suspect or to confiscate property, and a citizen does not generally have the right to use any force whatsoever in making an arrest. Generally, the best way to deal with a suspected crime is to report it to the police.