Use case models are widely used for requirements engineering to capture functional and non-functional requirements, guide scenario-based design and validation, and to manage projects. Our tool for use case development and evolution supports re-organization (refactoring) of use case models as well as the extension of use case models to include new functional and non-functional requirements. The tool is based on a three-level metamodel covering the environment or context of a use case model, the structure of use cases, and the event or message-passing details of a scenario. In this paper, we describe the tool that we have developed, and demonstrate its application to a case study for bank teller machines (ATMs). We show that the concept of refactoring can be applied to use case models as an aid to their development and evolution. We are now working on a firm semantic foundation for use cases in order to verify the behaviour-preserving property of individual refactorings.