Progress in the fields of science and technology is again the reason for updat­ ing this text most changes occur in Chaps. 4, 7, 8 and 14; in the others we have made a number of minor improvements. In Chap. 4, Signal Precessing and Receivers, we have shortened the presen­ tation of maser and parametric amplifier front ends, which are no longer com­ monly used as microwave receivers in radio astronomy. Instead, we have ex­ panded the presentation of cooled transistor and superconducting front ends, and we have added a short section on coherent multi-beam receiver systems. The chapter titled Observational Methods now appears as Chap. 7, and mainly addresses single dish observations. Chapter 8 now concerns Interfer­ ometers and Aperture Synthesis. Aperture synthesis has become the single important imaging technique in observational astronomy, and its use most has spread from the radio frequency range into other wavelength bands pro­ viding the only general method available for obtaining images of extremely high resolution and quality. The discussion of the foundations for aperture synthesis therefore have been extended and hopefully improved. Both the hardware and the software instrumentation is now collected in this chapter.