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 dependability, 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.