Part I of this book is devoted to the fundamentals of communication protocols, describing their working principles and implicitly also those of computer networks. Part II deals with the description of communication proto cols, offering an overview of the various formal methods, the essence of protocol engineering. Part III covers the protocol life cycle and the most important development stages, presenting the reader with approaches for systematic protocol design, with various verification methods, with the main implementation techniques, and with strategies for their testing, in particular with conformance and interoperability tests, and the test description language TTCN.