Gears fail or are damaged on occasion by impact fatigue loading. This study involves an evaluation of the resistance of commonly used gear steels to impact fatigue failure. A specially designed impact fatigue specimen and testing device were used to evaluate nine carburizing steels (SAE 4118, 4620, 4626, 4718, 4817, 8620, 8822, EX-1, and the European 16MnCr5) and three nitriding steels (Nitralloy G, Nitralloy N, and 5Ni-2Al). The relative impact fatigue resistances of the steels are discussed as well as the influences of such heat treatment variables as high carbon potential, high temperature carburizing, and refrigeration. In general, impact fatigue resistance was independent of core strength but improved with increasing nickel content in the nitrided steels as well as the carburized steels. Lowering retained austenite by refrigeration was found to be detrimental to impact fatigue resistance.