Quasars and active galaxies are the most powerful emitters of radiation in the universe. Modern radio telescope arrays have shown that the ultimate energy source resides in the central few parsecs of the galactic nucleus, and powers the emitting regions by way of two oppositely-directed relativistic jets. This volume presents the latest observations and theories of these remarkable objects. Topics discussed include superluminal motions, the physics of jets and shock fronts in jets, related optical observations and cosmic evolution. Particular attention is given to the 'unified theories' which attempt to show that many of the phenomena in powerful extragalactic objects are different aspects of a single basic mechanism, the main difference in their appearance being due to their different orientations with respect to the observer. The book is based on papers presented at a workshop in October 1989 held at the National Radio Astronomy Observatory's Array Operations Center in Socorro, New Mexico, and attended by about 60 astronomers from around the world. It provides an up-to-date review of the subject and is supplemented with a comprehensive bibliography. It should serve as a useful introduction and reference for professional astronomers and graduate students whose research interests include active galactic nuclei, quasars, and astrophysical fluid dynamics.