fifteen countries in Scandinavia, Europe, Asia, Australia, and U.S.A. All of them came to Stockholm primarily because they recognize the growing im portance of networks as complex systems, and their home institutions do not offer any systematic lectures on this topic. The Networks Course was originally initiated jointly by the Summer University of Southern Stockholm Foundation and the County Council of Stockholm, the Swedish Aviation Administration, the Swedish National Road Administration, the Swedish Post, the Swedish State Railways, and Telia AB. They have all served as joint sponsors and hosts for the Course. In the year 1993 the Course also was sponsored by the Swedish Transport and Communications Research Board. All these organizations have supported the publication of a series of key lectures from the Course, to be released as a single volume entitled Networks in Action. It is the ambition of the Foundation to create continuity in its activities for the future. The board has proposed to its principals to take a decision in this direction. It is my expectation that this will be the case for the Networks Course from 1995. This book will then serve as a basic reference for use in an era when the topic of Communication-Networks will be included on a permanent basis in the Summer University's agenda.