The dependability of computing services will become increasingly important in the 90s and beyond. This paper proposes a small number of basic concepts that can be used to explain the architecture of present and future fault-tolerant distributed systems and discusses a list of architectural issues that we find useful to consider when designing or examining such systems. For each issue we present known solutions and design alternatives, we discuss their relative merits and we give examples of systems which adopt one approach or the other. The aim is to introduce some order in the complex discipline of designing and understanding fault-tolerant distributed systems.