Confusion noise due to extragalactic sources is a fundamental astrophysical limitation for experiments aimed at accurately determining the power spectrum of the Cosmic Microwave Background (CMB) down to arcmin angular scales and with a sensitivity $Delta T/T simeq 10^{-6}$. At frequencies $lsim 200-300$ GHz, the most relevant extragalactic foreground hampering the detection of intrinsic CMB anisotropies is constituted by radio loud Active Galactic Nuclei (AGN), including ``flat--spectrum radiogalaxies, quasars, BL-LACs and blazars. We review our present understanding of astrophysical properties, spectra, and number counts of the above classes of sources. We also study the angular power spectrum of fluctuations due both to Poisson distributed and clustered radio sources and give preliminary predictions on the power spectrum of their polarized components. Furthermore, we discuss the capabilities of future space missions (NASAs MAP, Bennett et al. 1995; ESAs Planck Surveyor, Bersanelli et al. 1996) in studying bright radio sources over an almost unexplored frequency interval where spectral signatures, essential for the understanding of the physical processes, show up.
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