This paper presents a framework for controlling the evolution of complex software systems concurrently developed by teams of software engineers. A general technique for fine-grained revision control of hierarchically structured information, such as programs and documents, is described and evaluated. All levels in the hierarchy are revision controlled, leaves as well as branch nodes. The technique supports sharing of unchanged nodes among revisions, automatic change propagation, and change-oriented representation of differences. Its use in a software development environment is presented, facilitating optimistic check-out of revisions and alternatives, check-in with incremental merge support, visualization of change propagation, and an integrated flexible diff-ing technique providing group awareness for team members.