A distributed system consists of cooperating interactive components running concurrently and exchanging information by communication. The development of such systems needs a formal design process that step by step transforms an abstract specification into a concrete one by systematic refinement. The book provides definitions, concepts, and examples for correctness preserving refinement techniques explicating the underlying design decisions. The refinement rules are based on specifications which model communication histories with streams. The presentation covers state refinement, property refinement, interface refinement, architecture refinement, and communication refinement. It focusses on state refinement which constitutes the step from a history based to a state based specification of a component. The other notions of refinement are presented particularly regarding their relationship to state refinement.