The UML activity diagram is a general purpose workflow diagram used to model sequential aspects of a system. The activity diagram can be used to model Use Case scenarios in the system, or to model the dynamic behavior of a class as it moves from state to state, or the control flow of a class method.

The activity diagram is one of five diagrams in UML that are used to model the dynamic aspects of the system. Whereas sequence and collaboration diagrams are used to model the flow of control from object to object, Activity diagrams are used to model the flow of control from activity to activity. An activity is an on-going execution within a state machine that results in some action, which in turn may change the state of the system or return a value.

The activity diagram is very similar to a flow chart, except that it can also model concurrent, parallel flow.