The primary function of the cardiopulmonary system is to provide blood flow (and oxygen) in quantities sufficient to support the metabolic needs of the body. The capacity of the cardiopulmonary system to fulfill this function is maximally stressed when an individual’s metabolic rate is increased, a condition that occurs most commonly during physical activity/exercise. A number of physiological changes accompany and facilitate the accommodation of the circulatory system to the hemodynamic demands of exercise (Figure 1). In normal individuals, these changes (which during upright exercise include a tripling of the resting heart rate, a 60% reduction in systemic and pulmonary vascular resistance, and a 50% increase in stroke volume) can ultimately produce a 5-fold increase in cardiac output. The increase in cardiac output is accompanied by enhanced ventricular preload (as the ventricles move up their Starling curves to accommodate the increased workload), a doubling of systolic and mean pulmonary artery pressures (most of the increase in pulmonary artery pressures is due to the concomitant rise in left-sided filling pressures; the increase in transpulmonary pressure gradient is relatively small), and a more modest increase in systemic arterial pressures. 1–4 Congenital heart disease (CHD) may, in a variety of ways and to a variable extent, adversely affect these hemodynamic adaptations. For instance, patients with a Fontan procedure lack a pulmonary ventricle. They therefore cannot increase their pulmonary blood flow and pressures normally (and consequently cannot maintain their ventricular preload and systemic blood flow) during exercise. 5 Patients with tetralogy of Fallot and other CHDs often have congenital and/or acquired abnormalities of their pulmonary vasculature and therefore may be unable to reduce their pulmonary vascular resistance normally. Patients with complex CHD often have sinus node dysfunction and may be incapable of developing a normal heart rate (HR) response to exercise. 6 Ventricular dysfunction, residual shunts, valvular disorders, and associated pulmonary and skeletal muscle disorders may also impair the cardiopulmonary response to exercise. An evaluation of a CHD patient’s ability to exercise can therefore impart important information on the health of a child’s cardiopulmonary system and provide valuable insights into the factors that might be limiting a child’s ability to perform physical activities. The assessment of a child’s or adolescent’s exercise function, however, poses unique challenges related to the patient’s size and maturity. In addition, the dramatic changes that occur in the cardiopulmonary and musculoskeletal systems during the pediatric years complicate the interpretation of data acquired during these assessments. These considerations must be taken into account when children with CHD are evaluated. Most of the clinical tests employed by the pediatric cardiologist assess the cardiopulmonary system when the patient is at rest. Although valuable, these tests do not necessarily predict the manner in which the cardiopulmonary system will respond to the demands of exercise, nor do they reliably inform the clinician about a patient’s true capacity to perform physical activities. To acquire this information, assessments of exercise function must be undertaken. A number of tools are available to the clinician seeking to address this issue. The strengths and limitations of these tools will now be reviewed.