Cardiopulmonary exercise testing (CPET) in pediatric patients differs in many aspects from the tests as performed in adults. Children's cardiopulmonary responses during exercise testing present different characteristics, particularly indices of respiratory gas exchange (e.g. oxygen uptake, ventilation and ventilatory efficiency), which are essential in interpreting hemodynamic data. Diseases that are associated with myocardial ischemia are very rare in children. Important indications for CPET in children are the evaluation of exercise capacity and the non-invasive identification of pathologic features. In this article we will review the methodology, and clinical concepts exercise testing and interpretation of respiratory gas-exchange during exercise in children with congenital heart disease.