This book describes the design philosophy and concepts of a novel distributed fault-tolerant architecture developed by the Delta-4 project, a five-nation, thirteen-partner project investigating the achievement of dependability in open distributed systems, including real-time systems. The architecture features a dependable communication system using standard local area network technology, a distributed object-oriented application support environment, use of multicast or group communication protocols, and built-in support for user-transparent fault-tolerance. The book opens with an overview of the architecture and its objectives, then describes the concepts of de- pendability, real-time and distributed fault-tolerance and the design of each of the major architectural components, including the Delta-4 applications support environment and the open and extra performance variants of the architecture. Work is described in the field of security, on intrusion-tolerance and tolerance of software design faults by diversified design. Finally, there is an overview of the validation activities carried out on the architecture. The book gives an insight into state-of-the-art techniques for providing both dependable and real-time service in distributed systems.