The study of the Universe in the radio part of the electromagnetic spectrum. There is a wide radio window covering wavelengths from about 1 mm to 30 m, almost all of it accessible from ground-based observatories, both day and night. Emission mechanisms are either thermal (black-body radiation, free–free transition) or non-thermal (synchrotron radiation or masers). Maser sources are also common. Study of emission and absorption lines from molecules provides information about conditions in interstellar space. The 21-cm hydrogen line has proved to be a particularly valuable probe of the structure of our Galaxy and others.

Radio astronomy began in the 1930s with the pioneering work of K. G. Jansky and G. Reber, but it was not until after World War II that major research groups were set up. The first sources to be identified were the Sun, the Milky Way, and supernova remnants such as the Crab Nebula. As radio telescopes improved in sensitivity and resolution, radio galaxies and quasars were identified. New understanding of the late stages of stellar evolution came with the discovery of pulsars. The counting of radio sources of different brightnesses (and hence presumably different distances) has helped astronomers understand how the Universe has evolved, and the discovery of the cosmic microwave background has provided direct support for the Big Bang theory of cosmology.

Radio telescopes employ several different techniques to collect the very weak radio waves coming from the sky. The most familiar is the circular parabolic dish that gathers radio waves and brings them to a focus, but the simplest radio telescopes may consist of little more than a metal rod (a dipole antenna). Several antennas or telescopes may be used together as an interferometer to achieve high resolution, notably in the techniques of aperture synthesis and very long baseline interferometry.