Carrier communication over power lines, like carrier traffic over postal lines, has a tradition of about 50 years. It has become a vital element in electric system operation. Being restricted to power networks, it has a comparatively limited scope. Nevertheless, great efforts are being ex­ to keep pace with the demands made pended on technical advancement of both power and communication engineering. As in previous editions of this book, it is endeavored in the fourth edition to familiarize the man in the field with the fundamental concepts on which this technique and its practical application are based. The physical and electrical characteristics of the equipment involved are de­ scribed in a general way. For the sake of clarity, the author has refrained from giving a detailed account or illustrations of special versions, espe­ cially as the differences between the equipment supplied by the various manufacturers are so small. This is due to the use of transistors instead of tubes and to the universally adopted plug-in module construction technique. In the load dispatching plants of power supply networks, tele­ metering systems are turning more and more to data engineering methods and equipment to lighten the task of network management. Consequently, at certain points in the text, topics concerning remote data processing are treated, emphasis being placed on transmission channels operating at a considerably higher speed than was usual 10 years ago.