Cardiopulmonary exercise testing (CPET) is a valuable tool for assessing and managing patients with heart failure (HF). It measures respiratory and cardiac responses to exercise, providing data on oxygen uptake (VO2 max) and the relationship between minute ventilation (VE) and carbon dioxide excretion (VCO2). These parameters help classify patients into categories that predict prognosis, with a VO2 < 14 mL/kg/min and VE/VCO2 slope >35 indicating poor prognosis. CPET has been used in drug trials to evaluate complex physiological responses, such as with angiotensin-converting enzyme inhibitors. Chronic HF and reduced physical activity are associated with increased BNP levels, and cardiac rehabilitation can reduce BNP and increase VO2. MicroRNA measurements can also assess skeletal muscle status and changes during rehabilitation. However, CPET requires specialized technology, and simpler methods for measuring physical activity are needed. Recent advances in portable CPET equipment and smart watches can provide valuable information about cardiorespiratory function, detect unexpected clinical problems, and monitor treatment responses. This review discusses the principles, methods, clinical applications, and prognostic implications of CPET in HF patients.Cardiopulmonary exercise testing (CPET) is a valuable tool for assessing and managing patients with heart failure (HF). It measures respiratory and cardiac responses to exercise, providing data on oxygen uptake (VO2 max) and the relationship between minute ventilation (VE) and carbon dioxide excretion (VCO2). These parameters help classify patients into categories that predict prognosis, with a VO2 < 14 mL/kg/min and VE/VCO2 slope >35 indicating poor prognosis. CPET has been used in drug trials to evaluate complex physiological responses, such as with angiotensin-converting enzyme inhibitors. Chronic HF and reduced physical activity are associated with increased BNP levels, and cardiac rehabilitation can reduce BNP and increase VO2. MicroRNA measurements can also assess skeletal muscle status and changes during rehabilitation. However, CPET requires specialized technology, and simpler methods for measuring physical activity are needed. Recent advances in portable CPET equipment and smart watches can provide valuable information about cardiorespiratory function, detect unexpected clinical problems, and monitor treatment responses. This review discusses the principles, methods, clinical applications, and prognostic implications of CPET in HF patients.