This paper discusses schema evolution in software engineering databases. After a study of existing approaches, we show that these approaches do not satisfy software engineering requirements. Then, we present our model, which supports multiple schema compositions and multiple evolution policies, each application being free to define its evolution strategy. Management of our system is based on class versioning. The consistency of the database and the various evolution policies are controlled by consistency constraints. The schema composition uses software configuration techniques and evolution policy definition uses the capability of the active database of the Adele system.