One of the salient symptoms of patients with heart failure with reduced ejection fraction (HFrEF) is a limited exercise capacity. The diminished exercise capacity is the result of both intrinsic cardiac dysfunction as well as the peripheral response to chronic congestion and reduced cardiac output. We will review the factors contributing to reduced exercise capacity in these patients. Exercise performance of heart failure patients is best quantified by measurement of peak oxygen consumption (Peak VO2). This measurement along with the ventilatory response to exercise have been shown to have excellent prognostic value. Many of the peripheral muscle changes associated with heart failure may be partly attributed to deconditioning. Exercise can be therapeutic for heart failure patients and data to support cardiac rehabilitation in this patient population will be reviewed. As interpretation of cardiopulmonary exercise (CPET) data can be challenging to the general cardiologist, we will also present an easy guide to CPET interpretation.