A specific form of UserScenarios, formally documented with UML. Similar to a task analysis, a UseCase is precise documentation of a user's tasks as well as all the possible variations including edge cases. The usecase often is a two column affair, with the user behavior on teh left, system on the right. User does X, system reacts Y, user then does Z or B. If Z, system prints message and requests Q. If B, system does C. And so on. Tedious but effective. Used in the Rational methodology.