The specific aims for the study are 1) to determine if aerobic exercise enhances cognition for older adults with mild cognitive impairment, 2) to evaluate whether change in insulin sensitivity predicts cognitive performance for subjects in the aerobic exercise group, and 3) to relate exercise effects on insulin sensitivity and cognition to growth factor expression, inflammatory markers, catecholamines, beta-amyloid, and body-fat composition. Using a randomized controlled parallel group design, 40 sedentary older adults diagnosed with mild cognitive impairment will participate in a 6-month supervised protocol of either aerobic exercise or stretching. Cognitive testing and blood collection will occur at baseline, and months 3 and 6. Before and after the 6-month intervention, insulin sensitivity (via hyperinsulinemic-euglycemic clamp), maximum aerobic capacity (via respiratory gas exchange and cardiopulmonary fitness measures), and body fat composition (via DEXA scan) and distribution (via CT scan) will be assessed for all subjects. The results of this study may provide support for a relatively simple and inexpensive treatment strategy that specifically targets many of the health factors that directly influence risk of cognitive decline for older adults.