Within the UML (Universal Modeling Language) a number of modeling techniques can be used to document knowledge needed to build a system. One type of model is a use case. A use case is a scenario that describes the interaction between an actor and the system being modeled. Actors are people or other systems that interact with the system to achieve a desired goal. This interaction is represented as a sequence of steps to carry out an action. Each interaction between actor and system is a separate use case.

Use cases are meant to capture what the system needs to do; other UML techniques illustrate how the system will meet these requirements. The purpose of use cases in the requirements stage is to build a system model that is understandable by both the developers and the customers.

Although use cases are initially developed to capture requirements for the system, they can be employed during several stages of development. Estimating the work effort, validating the system design, testing the system, and developing system documentation can all take advantage of the knowledge that use cases contain.

Use case diagrams visually illustrate the relationships between actors and the system. In UML a use case is drawn as an oval, and the actors are drawn as stick figures. Actors are connected to the use case with lines. Arrowheads on the lines are often used to indicate the initiator of the interaction. Use cases include a textual description of the scenario that describes the main flow of events, an exception flow that describes what could go wrong and how the system should handle the exception.