The purpose of this review is to see what facts of cosmological interest can be dredged from wavelengths of above a few cm. In order to deal with modern research, rather than ancient history, it will be necessary to cheat a little and concentrate on the discrete-source population, rather than genuine smooth backgrounds - a strategy adopted by many other speakers at this meeting. However, to do duty to the advertised title, we begin with a few comments about what a non-discrete background might actually mean, were it to exist. Following this, the concentration will be on radio galaxies: why are they active, and how has the degree of activity changed with redshift? The final sections attempt to liberate us from the shackle of the radio waveband altogether, and to ask what general conclusions may be drawn about stellar evolution and galaxy formation from optical/IR data on high-redshift radio galaxies.
Notation: the Hubble constant, where quoted explicitly, is given in the form h = H0 / 100 kms-1Mpc-1. If not otherwise specified, = 1 and h = 0.5 are assumed.