CMB surveys provide, for free, blindly selected samples of extragalactic radio sources at much higher frequencies than traditional radio surveys. Next-generation, ground-based CMB experiments with arcmin resolution at mm wavelengths will provide samples of thousands radio sources allowing the investigation of the evolutionary properties of blazar populations, the study of the earliest and latest stages of radio activity, the discovery of rare phenomena and of new transient sources and events. Space-borne experiments will extend to sub-mm wavelengths the determinations of the SEDs of many hundreds of blazars, in temperature and in polarization, allowing us to investigate the flow and the structure of relativistic jets close to their base, and the electron acceleration mechanisms. A real breakthrough will be achieved in the caracterization of the polarization properties. The first direct counts in polarization will be obtained, enabling a solid assessment of the extra-galactic source contamination of CMB maps and allowing us to understand structure and intensity of magnetic fields, particle densities and structures of emitting regions close to the base of the jet.
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