Exercise is of both physical and psychological benefit in children with congenital heart disease, leading to improved cardiovascular fitness and better quality of life. In such children, however, capacity for exercise is often impaired, and there is a risk of exercise-induced mortality or morbidity. We have reviewed the abnormal cardiovascular response to exercise in children with congenital heart defects and postoperative residua, and the role of exercise testing and training in their diagnosis and treatment. Some patients should be excluded absolutely from all but mild regular exercise because of known high risk; for example, those with severe aortic or subaortic stenosis. Other patients should be encouraged to participate in sports without any limitation; for example, those with small left-to-right shunts or mild valvar regurgitation. In many patients strict recommendations cannot be made, and one must consider the individual, the lesion and its hemodynamic implications, and the type and level of exercise contemplated. Children with congenital heart disease should be encouraged to participate in exercise and recreational sport within the limits provided by their cardiovascular defect. Understanding the pathophysiology of these defects, and knowledge of the risk of exercise in certain conditions, will allow the physician to make sensible recommendations for participation in exercise by individual patients.