Exercise intolerance is a major burden for patients with complex congenital heart disease (CHD), significantly affecting their quality of life. Cardiopulmonary exercise testing provides a reliable tool both for assessing exercise capacity of CHD patients and for risk stratification and is becoming part of the routine clinical assessment of these patients. Exercise has an effect on the muscular, metabolic and circulatory systems. While exercise training has been widely studied in chronic heart failure, its efficacy in adults with CHD remain unknown. The investigators hypothesize that structured exercise training will improve exercise intolerance, in particular peak VO2. The aim of this multicenter, randomized study is to evaluate the impact of structured exercise training on exercise intolerance in patients with complex CHD.