 In the US, increased length of life and reduced morbidity and mortality have resulted in a growing number of older adults, the demographic time bomb often referred to in discussions of public policy. According to the Census Bureau, the population aged 65 and over will double in size within the next 25 years. Moreover, these older adults will live healthier lives than their predecessors. While this increased length of a healthy life is an undeniable societal benefit, it brings with it a major societal problem: an epidemic of aging-related cognitive decline. The need to develop interventions to address this growing problem is urgent. Aging-related cognitive dysfunction is not diffuse; rather it targets selected brain areas, in particular the frontal lobes and the hippocampal formation. The separate but interconnected subregions of the hippocampus are differentially vulnerable to pathogenic mechanisms, including the normal aging process. A range of in vivo and post-mortem studies have converged on the dentate gyrus DG as the hippocampal subregion differentially targeted by the aging process. As with pathogenic processes, any intervention that improves brain function does so with regional selectivity. One such intervention is physical exercise, which has been shown to improve both frontal lobe and hippocampal function. Using a high-resolution variant of functional magnetic resonance imaging fMRI, the investigators have demonstrated that aerobic training selectively benefitted DG function both humans and mice. In addition, improvement in DG function was associated with improved performance on a word list learning task but not in tasks conventionally thought to be frontal lobe dependent. The human part of the study had significant shortcomings, however: it was small N 11, lacked a control group, enrolled only young subjects age 20-45 years, and employed only a limited neuropsychological testing battery. The overall goal of this proposal is to use the high-resolution variant of fMRI to test the hypothesis that aerobic training will induce improvements in DG function in a sample of younger age 20-35 and older 50-65 adults, assigned randomly to an active training condition or wait list control group. The investigators will use more comprehensive neuropsychological testing to examine the relationship between changes in DG function and selected cognitive capacities. Taken together with the observation that normal aging differentially targets the DG, this research program will establish that physical exercise is an effective approach for ameliorating the insidious cognitive slide that occurs in aging. Thus, the potential significance of this application is substantial. 