Medically prescribed and supervised exercise as part of a comprehensive rehabilitation program is a well accepted standard for care throughout the world for cardiac patients, particularly those following an acute myocardial infarction or coronary revascularization procedure. Early exercise testing for such patients is now considered an essential component of such a program for risk stratification to identify those needing additional diagnostic procedures and therapeutic care and for prescribing appropriate physical activity. Once patient risk is determined only moderate to high risk patients should be continued on daily telemetric ECG monitoring. There is substantial evidence that a minimum of 2-3 months and ideally at least 6 months of medically supervised exercise conditioning will result in cardiovascular adaptations which will optimize functional capacity and help the patient to resume active, productive lives. It is hoped that this period also will provide reinforcement to motivate the patient to continue physical conditioning and modification of other risk factors for life. However, this has not been scientifically documented. Well documented physiological adaptations from the conditioning program include peripheral changes involving the active skeletal muscle. These changes are the principal contributors to demonstrated increase in maximal aerobic capacity, endurance, and muscular strength. High intensity, longer term exercise programs also may favorably improve myocardial function in selected cardiac patients. Although experimental animal work suggests that myocardial vascular supply may also be augmented by endurance exercise training, this has not been adequately confirmed in patients with coronary heart disease. Instead, a reduction in myocardial oxygen demands through lowering of heart rate and systolic blood pressure during submaximal physical exertion is more likely to contribute to a restoration in the balance between myocardial oxygen on supply and demand in patients with underlying ischemia. There also is substantial evidence that endurance exercise conditioning can favorably alter several other risk factors for coronary heart disease. This theoretically should have a favorable impact on the underlying atherosclerotic process, but it remains to be proven. Although it is generally assumed by both health professionals and exercisers that exercise conditioning promotes psychosocial well-being, it has not as yet been possible to demonstrate this by controlled studies. There also has not been a definitive randomized clinical trial on the independent effect of exercise in prevention of recurrent coronary events in patients recovering from myocardial infarction, coronary bypass graft surgery, or angina pectoris. Pooling of data from existing controlled randomized trials involving patients recovering from an acute myocardial infarction provides supportive evidence that a comprehensive cardiac rehabilitation program can reduce premature mortality from cardiovascular events in such patients, but probably not nonfatal reinfarctions. The risk of major cardiovascular events and fatality associated with supervised cardiac rehabilitation programs appears to be quite low. It is concluded that there is sufficient existing evidence supporting the importance of exercise in the rehabilitation of the cardiac patient even though vigorous scientific proof is still lacking to document all of the postulated benefits.