The V/Vmax test for quasars and the counts of radio sources show that the most powerful extragalactic radio sources exhibit strong evolutionary changes with cosmological epoch (see M. Schmidt and J.V. Wall et al., this volume). It should be emphasised that these are very large changes indeed. Schmidt, for example, has shown that for the world model with Ω = 0, evolution functions of the form F(t)∝exp(-10t/to) can account for the observations, to being the present epoch and t cosmic time. Since the quasar population from which this law is derived extends at least to Z = 2.5, corresponding to t = to/3.5 if Ω = 0, the comoving space density of quasars at Z = 2.5 must have been about 1300 times greater than it is at the present epoch. It is not known whether or not this law continues to hold at larger redshifts but even if it does, the increase in comoving space density from Z = 2.5 to infinity is only a further factor of 17. Therefore the bulk of the evolution occurs within the range of observationally accessible redshifts. Notice that a characteristic time-scale for the decay of the quasar population of ≈ to/10 ≈ 109 years comes out of this analysis.