  Surveys at mm wavelengths emphasize rare classes of extragalactic radio
sources characterized by spectra keeping flat or inverted up to high
frequencies (such as blazars, GPS sources, advection dominated sources, or even
gamma-ray burst afterglows), which are are difficult to single out at lower
frequencies where the counts are dominated by far more numerous populations.
Such surveys may lead to the discovery of populations with extreme properties,
such as sources of the general GPS or blazar type but peaking at mm
wavelengths, whose existence is hinted by currently available data. At
relatively lower flux levels mm counts are dominated by high-redshift dusty
galaxies, such as those detected by SCUBA and MAMBO surveys. These data have
profound implications for our understanding of the formation and early
evolution of galaxies. High sensitivity surveys at cm/mm wavelengths may also
detect galaxy-scale Sunyaev-Zeldovich effects due to gas heated by central
active nuclei.
