The very first astronomical signal at radio wavelengths, by happy coincidence, was also detected at the Bell Telephone Laboratories; in 1932, Karl Jansky detected at 15 m wavelength radio emission, which he correctly identified as coming from the Galactic plane. Astronomers paid little attention. Observational radio astronomy did not really come into its own until after World War II (see, e.g., Hey, 1973, and Sullivan, 1984). It is now recognized as a powerful adjunct to optical astronomy, particularly in the study of low density cosmic matter and of energetic objects and phenomena such as quasars and the collimated jets seen in radio galaxies. We will look very briefly at radio sources later in this chapter, but most of it will be devoted to the tools and techniques of observational radio astronomy. Chapter 3 is designed to introduce the more specialized radio astronomical techniques used in studying the CBR; it is not intended to be a complete introduction to radio astronomy. For further details, readers may want to consult one or more of the following texts: Kraus (1986); Rohlfs (1986); and Christiansen and Högbom (1985). Interferometry is very fully treated by Thompson, Moran and Swenson (1986), and radio sources by Pacholczyk (1970) and Verschuur and Kellermann (1988), among others. The treatment of radio astronomy in this book is closest to the work of Rohlfs.