We briefly review the extant human and animal literature on the influence of fitness training on brain, cognition and performance. The animal research provides clear support for neurochemical and structural changes in brain with fitness training. The human literature suggests reliable but process specific changes in cognition with fitness training for young and old adults. We describe a research program which examines the influence of aerobic fitness training on the functional activity of the human using event-related functional magnetic resonance imaging, of humans in fitness interventions.