The radio and optical characteristics of quasi-stellar radio sources are described and contrasted with those of radio galaxies. It is shown how the synchrotron hypothesis of the radio emission from those objects can explain many of the features of their radio spectra. Once a consistent model for the evolution of quasi-stellar radio sources and radio galaxies is sought many difficulties are encountered, particularly during the early stages when the radiation density is very great. However, the present observational data allow several important conclusions to be drawn about the evolution of quasi-stellar radio sources and radio galaxies in general.