Historically, patients with heart failure were advised to avoid exertion due to fear that the added myocardial stress would worsen heart function. However, between 1979 and 1991, five uncontrolled trials demonstrated that moderate exercise training can partially reverse the exercise intolerance common among patients with heart failure. Since 1991, numerous randomized exercise trials demonstrated a 12% to 33% improvement in exercise capacity, as measured by oxygen consumption. The mechanisms by which this improvement occurs are now being studied. Potential mechanisms include improved cardiac output due to increases in both peak stroke volume and reversal of chronotropic incompetence; improved regional blood flow to the metabolically more active skeletal muscle; and partial reversal of skeletal muscle histochemical abnormalities such as increased oxidative enzymes and a “re-shift” in fiber type toward increased type I fibers. Improved sympathetic function has also been noted with exercise training, evidenced by decreased norepinephrine spillover and increased heart rate variability. Exercise training can be of benefit in selected patients with stable New York Heart Association class II or III heart failure.