It is a criminal offense to unlawfully restrain or falsely imprison a person. Laws vary from state to state, but the elements of false imprisonment are generally (1) willful detention; (2) without consent; and (3) without authority of law. The definitions and punishment for the criminal offense of unlawful restraint or false imprisonment are usually located in a state’s statutes. In many states there may also be civil liability for money damages in a lawsuit for the tort of false imprisonment.

The willful detention required for false imprisonment may result from acts or words with the intention to confine the plaintiff and prevent her from exercising her free will to leave. A person may be restrained without her consent by force, intimidation, or deception. Where it is alleged that a detention is effected by a threat, the plaintiff must demonstrate that the threat was such as would inspire in the threatened person a just fear of injury to her person, reputation, or property. But if a plaintiff voluntarily complies with a request to remain and establish his innocence, there is no cause of action for false imprisonment.

And if an arrest or detention is based on an arrest warrant or subpoena, the arrest or detention is presumed to have been made with legal authority. When the plaintiff is arrested or detained without a warrant, there is no presumption it was done with legal authority, and the defendant must prove there was legal authority.

But when the plaintiff is not arrested and is merely detained, there are some privileges that may apply and prevent the defendant from being liable for detaining the plaintiff:

• shopkeeper’s privilege—This privilege permits a shopkeeper to detain a person to investigate the ownership of property if the shopkeeper reasonably believes that the person has stolen or is attempting to steal store merchandise so long as the detention is in a reasonable manner and for a reasonable period of time.

• officer’s privilege—Under an exception to the warrant requirement, an officer is generally justified in briefly detaining an individual on less than probable cause for the purposes of investigating possibly-criminal behavior where the officer can point to specific and articulable facts, which, taken together with rational inferences from those facts, reasonably warrant the intrusion. The information provoking the officer's suspicions need not be based on his own personal observations, and may be based on an informant's tip which bears sufficient indicia of reliability to justify a stop. The same standards will apply whether the person detained is a pedestrian or is the occupant of an automobile.

• employer’s privilege—Employers act within their legal rights in investigating reasonably credible allegations of dishonesty of their employees.

Liability for false imprisonment may extend beyond those who willfully participate in detaining the complaining party to those who request or direct the detention. The first element of false imprisonment may be satisfied by conduct that is intended to cause someone to be detained, and in fact causes the detention—even when the actor does not participate in the detention. This is sometimes referred to as instigation of the false imprisonment.

When the alleged detention results from an unlawful arrest, to prove instigation a plaintiff must show that the defendant clearly directed or requested the arrest. As explained in the Restatement (Second) of Torts §45A cmt. c, "[i]n the case of an arrest, [instigation] is the equivalent, in words or conduct, of ‘Officer, arrest that man!’” To hold a third party liable for instigating the detention, then, the act of arrest must be made by the officer, not of his own volition, but to carry out the request of the defendant.

A private citizen who merely reports a crime and identifies the suspect to law enforcement authorities has not requested or directed the suspect's arrest and will not be liable for ins,tigating a subsequent false imprisonme,nt. A citizen has a clear legal right to report criminal misconduct to authorities, and from the mere exercise of this right the law will not permit the inference to be drawn that he `requested or directed' the arrest, though it be conceded that but for its exercise the arrest would never have been made. This is true even when the reporting party mistakenly identifies the wrong person.

And it is not enough for instigation that the informant has given information to the police about the commission of a crime, or has accused the other of committing it, so long as he leaves the decision to the police as to what shall be done about any arrest, without persuading or influencing them. But a defendant may be liable for instigating an unlawful arrest if he knowingly provides false information to law enforcement authorities resulting in the arrest.

In Montana, the criminal offense of unlawful restraint or false imprisonment is defined under Montana Code Annotated (MCA) § 45-5-301 and § 45-5-302. The statutes outline that a person commits the offense if they knowingly or purposely restrain another person without their consent and without legal authority. The elements of false imprisonment in Montana align with the general definition, requiring willful detention without consent and without legal authority. Penalties for these offenses can range from fines to imprisonment, depending on the severity of the restraint and whether it involved a minor. In addition to criminal penalties, Montana law also allows for civil liability for false imprisonment, where victims can sue for money damages. Defenses to false imprisonment in Montana include the shopkeeper's privilege, which allows for the detention of a suspected shoplifter under reasonable conditions, and the officer's privilege, which permits law enforcement to detain individuals based on reasonable suspicion of criminal activity. An employer's privilege also exists for detaining employees during investigations of credible allegations of dishonesty. For a third party to be liable for instigating false imprisonment, they must have directed or requested the detention, and merely reporting a crime does not constitute instigation. Providing false information to law enforcement that leads to an arrest, however, could result in liability for false imprisonment.