Recent studies have shown the encouraging effects of fitness training on human cognition, and brain structure and function. Such effects are of great interest both for what may they tell us about the nature of cognitive and brain aging and also for their public health implications. This study will test the hypotheses that: 1) improvements in aerobic fitness of older adults will lead to improved performance on a variety of cognitive processes, and especially those processes that are supported by frontal regions of the brain; 2) improvements in cognitive processes due to enhanced aerobic fitness will be visible on fMRI scans, and will be similar to those of young adults; 3) improvements in aerobic fitness, over the course of a 1 year intervention, will result in increases in gray and white matter volume.