In this paper we review the literature that has examined the nature and strength of the relation between physical fitness and cognitive performance in old age. We consider both cross-sectional studies, which have investigated differences in cognitive performance between discrete groups of high- and low- fit older adults, and training studies, which have examined the effect of exercise training regimens on cognitive performance. We do not consider the acute or short-term effects of a single bout of physical activity on cognitive performance. These data have been reviewed elsewhere [98] and are beyond the scope of this manuscript. Studies that examine the acute effects of a single bout of exercise stress are discussed only when they contribute directly to our understanding of general principles underlying the relationship between physical fitness and cognitive performance.