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Published online by Cambridge University Press: 26 January 2006
Nearly all discrete radio sources are extragalactic and lie atcosmological distances. Most sources stronger than ~1 mJy at1.4 GHz are powered by active galactic nuclei (AGNs), while themajority of fainter sources are in starburst galaxies. Recent all-skysurveys have detected most of this AGN population, and smaller surveysare now sensitive enough to probe the evolution of star formation inselected areas. Sensitive new radio surveys will complementfar-infrared, ultraviolet, and X-ray surveys now being made in “cosmicwindows” having exptionally low interstellar extinction. They willhelp define the evolution of star-formation and dust-shrouded AGNsover cosmological timescales.