I am very pleased to introduce this first edition of the Handbook on digital radio-relay systems, prepared by a group of experts from Radiocommunication Study Group 9 under the chairmanship of Dr. Rudolf Hecken (United States of America). The Handbook provides a detailed coverage of the state-of-the-art in present-day, high usage, digital microwave transmission equipment and offers some insight into present and future technological trends. It consists of 6 chapters which describe the basic principles, propagation and layout considerations, design parameters, link engineering, operation and maintenance of radio-relay systems. Many references are also provided that can be consulted, if needed, for additional details.