Heart failure is a common condition affecting between one and two percent of the population in Western society, although it is a surprisingly poorly defined and understood condition. Cardiopulmonary exercise testing (CPET) can be carried out safely in a wide range of patients with heart failure, although there are some contraindications and the exercise protocol may have to be adjusted. CPET has shed some light on the pathophysiology of heart failure. The observation that patients with heart failure can achieve higher VO2 values when walking as opposed to cycling is one argument against the haemodynamic hypothesis. The cardiopulmonary exercise test is considered the ‘gold standard’ when assessing a patient's exercise tolerance. The peak VO2 is a measure which has been validated in numerous heart failure populations, and a value of 14mls/kg/min is still used as a cut-off when considering patients for cardiac transplantation. There are limitations with the use of peak VO2 alone, and numerous other values derived from the exercise data are discussed, of which the VE/VCO2 slope is the best researched. In summary CPET is of great value in the assessment of patients with heart failure.