This book is written for computer programmers, analysts and scientists, as well as computer science students, as an introduction to the principles of distributed system design. The emphasis is placed on a clear understanding of the concepts, rather than on details; and the reader will learn about the structure of distributed systems, their problems, and approaches to their design and development. The reader should have a basic knowledge of computer systems and be familiar with modular design principles for software development. He should also be aware of present-day remote-access and distributed computer applications. The book consists of three parts which deal with principles of distributed systems, communications architecture and prools, and formal description techniques. The first part serves as an introduction to the broad meaning of "distributed system". We give examples, try to define terms, and discuss the problems that arise in the context of parallel and distributed processing. The second part presents the typical layered prool architecture of distributed systems, and discusses problems of compatibility and interworking between heterogeneous computer systems. The principles of the lower layer functions and prools are explained in some detail, including link layer prools and network transmission services. The third part deals with specification issues. The role of specifications in the design of distributed systems is explained in general, and formal methods for the specification, analysis and implementation of distributed systems are discussed.