Findings from three research paradigms that employed aerobic exercise as an independent variable were used to test the hypothesis that aerobic exercise improves cognitive-neuropsychological functioning. The research paradigms were animal intervention studies, cross-sectional human studies, and human intervention studies. Results from studies of animals, usually rodents, provide consistent evidence that aerobic fitness is associated with improved neurobiological and behavioral functioning. Cross-sectional studies with humans indicate a strong positive association between physical activity level and cognitive-neuropsychological performance. However, results from these studies must be interpreted cautiously, as individuals who elect to exercise or not exercise may differ on other variables that could influence cognitive-neuropsychological performance. To date, human intervention studies have not consistently demonstrated cognitive-neuropsychological improvements following exercise training. To satisfactorily test the exercise/cognition hypothesis with humans, carefully controlled intervention studies that last longer than those previously employed are needed.