A requirement is "a condition of capability needed by a user to solve a problem or achieve an objective" [IEEE Std 610. 12-1990]. Use cases are a way to document requirement using a pictures (use case diagram) and text to describe the functionality. A use case diagram is part of UML.

A use case describes a sequence of actions that an outside actor interacts with in an application. Similar scenarios make up a use case. A use case is represented by an oval that contains a short phrase that describes an action done by the system. An actor is represented by a stick figure. Actors define roles, not individuals. Lines are drawn between use cases and actors or other use cases to show interaction. The default interaction is <<communication>> between the two elements of the diagram. Other interactions are described by the <<include>> and <<extend>> relationships, where one use case includes or extends the functionality of another use case.

A flow of events is used to textually describe the possible events in the use case. First, preconditions indicate what state the system should be in before the use case can be started. Secondly, a main flow of events is listed in the order they should be implemented. Any subflows or alternative flows are mentioned in the main flow where there is the possibility of occurance. Third, subflows are listed. Subflows are used to break down the main flow into smaller pieces. Last, alternative flows are described. Alternative flows describe errors and other behavior that could interrupt the main or subflows.