The criminal offense of unlawful assembly generally means the assembly of multiple persons resulting in conduct that (1) is intended to commit a breach of the peace or other unlawful act; (2) creates an immediate danger of damage to property or injury to persons; (3) substantially obstructs law enforcement or other governmental functions or services; (4) is tumultuous or violent and likely to cause public alarm; or (5) deprives any person of a legal right or disturbs any person in the enjoyment of a legal right by using force or the threat of force.

Unlawful assembly laws vary from state to state and some states have related offenses such as rioting, riots and routs, participating in a riot, inciting a riot, failing to disperse, and disturbing the peace. The criminal offense of unlawful assembly may be included in the definition of one or more of these offenses or may be a separate criminal offense. These laws are generally located in a state’s statutes—often in the penal or criminal code.

In Washington State, the criminal offense of unlawful assembly is addressed under RCW 9A.84.020, which defines 'Riot.' According to this statute, a 'riot' involves an assembly of three or more persons acting with a common purpose to intentionally cause a public disturbance involving an act or acts of violence by one or more persons in the group that are likely to substantially harm persons or property, or substantially obstruct law enforcement or other governmental functions. The law makes it a gross misdemeanor to participate in a riot. Additionally, Washington law under RCW 9A.84.030 prohibits failure to disperse, which occurs when someone knowingly joins or remains in a group of three or more persons where one or more are engaging in or are about to engage in rioting, and he or she has been ordered to disperse by a peace officer. This is also a gross misdemeanor. These statutes reflect Washington's approach to handling situations that may involve unlawful assembly, with specific emphasis on the prevention of public disturbances and the protection of persons and property.