Many patients undergoing CPET have underlying pulmonary or cardiac disease and wish to improve their quality of life with some form of exercise. Structured rehabilitation programs both for pulmonary and cardiac patients are designed to improve exercise capacity. CPET is valuable to document objective progress in these patients undergoing rehabilitation. In cardiac patients, CPET can identify those patients with exercise hypotension, dysrhythmias, or ischemia to initiate further medical stabilization prior to embarking on the exercise portion of their program. For those with a benign exercise response, the test will establish an appropriate level of training intensity. Improvement in aerobic capacity through augmentation of stroke volume is one benefit. Most pulmonary patients in pulmonary rehabilitation programs have COPD but may include patients with restrictive lung disease or pulmonary hypertension. A CPET prior to starting the rehabilitation helps to detect underlying cardiac disease while also providing realistic levels of exercise during the program. Although resting pulmonary function tests and VO2 max do not change appreciably, patients are able to exercise longer with less dyspnea and improve their daily activities by experiencing greater efficiency in lung mechanics and general biomechanics. This chapter reviews these data in light of CPET.