Radio Astronomy has added greatly to our understanding of the structure and dynamics of the universe. The cosmic microwave background radiation, considered a relic of the explosion at the beginning of the universe some 18 billion years ago, is one of the most powerful aids in determining these features of the universe. This paper is about the discovery of the cosmic microwave background radiation. It starts with a section on radio astronomical measuring techniques. This is followed by the history of the detection of the background radiation, its identification, and finally by a summary of our present knowledge of its properties. II. RADIO ASTRONOMICAL METHOD S A radio telescope pointing at the sky receives radiation not only from space ,but also from other sources including the ground, the earth’s atmosphere, and the components of the radio telescope itself. The 20-foot horn-reflector antenna at Bell Laboratories (Fig. 1) which was used to discover the cosmic microwave background radiation was particularly suited to distinguish this weak, uniform radiation from other, much stronger sources. In order to understand this measurement it is necessary to discuss the design and operation of a radio telescope, especially its two major components, the antenna and the radiometer 1 . a. Antennas An antenna collects radiation from a desired direction incident upon an area, called its collecting area, and focuses it on a receiver. An antenna is normally designed to maximize its response in the direction in which it is pointed and minimize its response in other directions. The 20-foot horn-reflector shown in Fig. 1 was built by A. B. Crawford and his associate? in 1960 to be used with an ultra low-noise communications receiver for signals bounced from the Echo satellite. It consists of a large expanding waveguide, or horn ,with an off-axis section parabolic reflector at the end. The focus of the paraboloid is located at the apex of the horn, so that a plane wave traveling along the axis of the paraboloid is focused into the receiver, or radiometer, at the apex of the horn. Its design emphasizes the rejection of radiation from the ground. It is easy to see