by the completion of large aperture synthesis radio telescopes, which have permitted a detailed mapping of radio sources, and by the development of very long base-line interferometry (VLBI), with 4 3 resolutions down to 10- - 10- arc sec, which for the first time has rendered possible radio investigations of the structures of the very compact radio sources found in galactic nuclei and quasars. The observations obtained with this new instrumentation, combined with the work in the optical, infrared and X-ray domain, have made great strides in recent years toward improved testing of radio source models. We feel that the material presented at the Institute represents a rather complete and comprehensive coverage of the present status of studies of non-thermal radio sources, though one is aware of rapid developments in this field of research. The various aspects of this exciting subject were covered in a series of lectures, pre­ sented in this volume, totaling 44 hours and in 14 topical seminars given by the participants. I wish to express my gratitude to the Scientific Affairs Divi­ sion of the North Atlantic Treaty Organization for the generous support given to the course. Sincere thanks are also due to Mr. L. Baldeschi for helping with the organization of the meeting and for drawing a number of figures contained in this volume; to Mrs. B. Mandel for the patient typing and help in the editing; and to Mr. R. Primavera for the photographic reproduction of part of the figures.