  We review the statistical properties of the main populations of radio
sources, as emerging from radio and millimeter sky surveys. Recent
determinations of local luminosity functions are presented and compared with
earlier estimates still in widespread use. A number of unresolved issues are
discussed. These include: the (possibly luminosity-dependent) decline of source
space densities at high redshifts; the possible dichotomies between
evolutionary properties of low- versus high-luminosity and of flat- versus
steep-spectrum AGN-powered radio sources; and the nature of sources accounting
for the upturn of source counts at sub-mJy levels. It is shown that
straightforward extrapolations of evolutionary models, accounting for both the
far-IR counts and redshift distributions of star-forming galaxies, match the
radio source counts at flux-density levels of tens of microJy remarkably well.
We consider the statistical properties of rare but physically very interesting
classes of sources, such as GHz Peak Spectrum and ADAF/ADIOS sources, and radio
afterglows of gamma-ray bursts. We also discuss the exploitation of large-area
radio surveys to investigate large scale structure through studies of
clustering and the Integrated Sachs-Wolfe effect. Finally we briefly describe
the potential of the new and forthcoming generations of radio telescopes. A
compendium of source counts at different frequencies is given in an appendix.
