The state diagram depicts the behavior of the system over time. A state diagram is typically drawn for every class in the system that contains significant dynamic behavior. The behavior of the class is modeled in terms of what states it is in at various times, what actions it performs while in various states, and when it transitions from state to state based on events in the system. The UML State diagram is based on the work of David Harel, and is sometimes referred to as a Harel Statechart.

States represent the conditions of objects at certain points in time. Events represent incidents that cause objects to transition from one state to another. Transition lines depict the movement from one state to another. Each transition line is labeled with the event that causes the transition. Actions occur when an object arrives in a state.