The process of aging is associated with numerous changes in all bodily systems that ultimately manifest in a decline in peak physiologic function. Preservation of functional ability in advanced age is predicated on maintenance of the 4 components of fitness-cardiorespiratory endurance, muscular strength and endurance, flexibility, and body composition-each of which show changes with aging that adversely impact functional ability. A requisite awareness and understanding of the effects of aging on the components of fitness and understanding of the role of exercise as intervention is of paramount importance. This review presents underlying changes and adaptations within each component of fitness and the effects on overall physiologic capacity from the more overt changes underlying declining aerobic capacity, pulmonary function, and flexibility, to the more latent changes such as demineralization of bone leading to changes in body composition and the contraction-specific decline in the force generating capacity of skeletal muscle. Because exercise and physical activity have been shown to attenuate or delay many of the agerelated changes in the components of fitness, recommendations and guidelines for exercise prescription are offered in the context of the these changes.