We discuss the role of order in building distributed systems. It is our belief that a "principle of event ordering" underlies the wide range of operating systems mechanisms that have been put forward for building robust distributed software. Stated concisely, this principle is that one achieves correct distributed behavior by ordering classes of distributed events that conflict with one another. By focusing on order, one can obtain simplified descriptions and convincingly correct solutions to problems that might otherwise have looked extremely complex. Moreover, we observe that there are a limited number of ways to obtain order, and that the choice made impacts greatly on performance.