The key requirement for open systems is that they be flexible, or recomposable. This suggests that they must first of all be composable. Object-oriented techniques help by allowing applications to be viewed as compositions of collaborating objects, but are limited in supporting other kinds of abstractions that may have finer or coarser granularity than objects. A composition language supports the technical requirements of a componentoriented development approach by shifting emphasis from programming and inheritance of classes to specification and composition of components. Objects are viewed as processes, and components are abstractions over the object space. An application is viewed as an explicit composition of software components. By making software architectures explicit and manipulable, we expect to better support application evolution and flexibility. In this position paper we will elaborate our requirements and outline a strategy for the design and implementation of a composition language for the development of open systems.