This chapter first discusses a number of software reuse and extension problems in current object-oriented languages. For this purpose, a change case for a simplified mail system is presented. Each evolution step in the change case consists of the addition or refinement of certain aspects to existing classes. These examples illustrate that both inheritance and aggregation mechanisms cannot adequately express certain aspects of evolving software. This deficiency manifests itself in the number of superfluous (method) definitions that are required to realize the change case. As a solution to these problems, the composition filters model is introduced. We evaluate the effectiveness of various language mechanisms in coping with evolving software as in the presented change case.