Avi Wigderson, Herbert H. Maass Professor in the School of Mathematics, gave a Friends Forum in October 2011, entitled Randomness and Pseudo-randomness. Is the universe inherently deterministic or probabilistic? Perhaps more importantly, can we tell the difference between the two? A computational theory of pseudo-randomness, developed in the past three decades, reveals (perhaps counter-intuitively) that very little is lost in such deterministic or weakly random worlds, since what seems like perfect randomness can be created deterministically. In this talk, Wigderson explains the main ideas and results of this theory. To learn more about the Friends, please visit the Friends of the Institute for Advanced Study.