In recent years aperture synthesis and interferometry have become very powerful tools in radioastromy. Investigation of distant galaxies, for example, have revealed structures with sizes of less than a kiloparsec. In general, the study of galaxies has benefited from the great power of these techniques. Radar applications have also dramatically increased their quality by using the interferometry principle. Tracking and airborne radar can w determine position and velocity of objects with a much higher accuracy. This book describes in the first six, short chapters the basics of interferometry and aperture synthesis. The following two, long chapters treat the aspects of radioastromical interferometers and radar applications of interferometry in great detail. The text offers readers a very good opportunity to familiarize themselves with the mathematical background of these very complex techniques. For researchers and students in radioastromy and electrical engineering.