The lack of energy conservation in an expanding cosmological space-time makes possible decay processes which are forbidden in Minkowski space-time. In addition, cosmological modes in general do not decay exponentially with time as shown by explicit quantum field evolution calculations. Furthermore, one-loop quantum corrections to the slow-roll classical inflaton evolution are explicitly derived. The infrared (superhorizon) contributions exhibit infrared enhancements and the ultraviolet (subhorizon) parts are completely specified by trace anomalies. Explicit formulas for the loop corrections to the effective inflaton potential and to the primordial power, source of the CMB anisotropies and seed of the large scale structure of the universe are given.