To discuss the theories of radio emission it is convenient to divide the sources into five different classes: (1) planets, (2) the sun, (3) galactic nebulosities, (4) galaxies taken as a whole, and (5) clusters in galaxies. About (1) and (2) I shall say little. It appears that the temperatures of Venus and Jupiter obtained from the thermal radio emission measures are in reasonable agreement with the temperatures determined by the older astronomical methods. No theories explaining the nonthermal emission from Jupiter have yet been developed, though there has been discussion of the possibility that some type of discharge mechanism leading to plasma oscillations is involved.