Published online by Cambridge University Press: 19 July 2016
Evidence is now accumulating that the most powerful extragalactic radio sources display a cutoff in comoving density at relatively low redshift. For both radio galaxies and quasars, the luminosity function falls by a factor ≥ 3 between z = 2 and 4. We describe the data which lead to this conclusion, and discuss the prospects for studying the behaviour of low-luminosity active galaxies at high redshift. The strong evolution of highly luminous radio-quiet quasars at z > 2 may be an artifact of gravitational lensing.