In this application the study team propose to conduct a study in which 270 sedentary but otherwise healthy and cognitively intact individuals in the 20-68 year age range are randomized to two training conditions, aerobic exercise and stretching/toning, to be completed at YMCAs and YMHAs in New York City. Participants will be assessed for aerobic capacity, cognitive task performance, and by structural MRI, resting cerebral blood flow scans (arterial spin labeling) and cognitive activation fMRI studies at study entry and after 6 months of training.