Clinicians who understand how the body responds to exercise, how aerobic training improves cardiovascular fitness, and the benefits and principles of prescribing aerobic exercise can effectively encourage patients to become active and optimize programs for those already active. Patients who are active at an early age and who continue to enjoy active lifestyles as adults will attenuate the normal losses in cardiovascular endurance, strength, and flexibility that accompany aging and sedentary living, thereby maintaining greater independence throughout their life spans.