The microJansky and nanoJansky population

We present a review of faint radio source populations, starting with constraints from the radio source counts over 7 dex in flux density and ∼1 dex in frequency, as well as from the angular-size and spectral-index vs. flux-density relations. We present possible extrapolations of the μJy source counts to nanoJy levels as they may be observed with the Square Kilometer Array (SKA). We review clues to the nature and evolution of faint radio sources from deep optical identification and spectroscopy programs, as well as from HST imaging. The faint radio source population is a mixture of starburst, post-starburst and elliptical galaxies. The microwave radio emission at μJy-mJy levels is caused by a combination of (nuclear) starbursts and weak AGN activity. Each of these radio source populations evolved strongly with cosmic epoch, likely driven by the strongly epoch dependent galaxy merger rate, which has been winding down since z≲0.5-1.0 when Λ took over the expansion.