We presented a modular model for developers of distributed systems where the exact ways in which interacting applications can be composed may be unforeseeable. Our theory provides two fundamental structuring units, processes and sessions, that can be developed using nested sequential, choice, and parallel composition. We investigated solutions to the problems of specifying, composing, reasoning about, and implementing distributed applications, through the use of processes and sessions. Combined with the theory of systematic process and session composition, our infrastructure tools, implemented using Java, allow a developer to create collaborative distributed applications on the Internet.