The purpose of this study is to (1) quantify cardiovascular anatomy and physiology using magnetic resonance imaging under both resting and exercise conditions in patients who have congenital heart disease and in age-matched normal volunteers, (2) use computer models to reproduce and simulate blood flow in these patients, and then (3) to combine the imaging data and computer models to estimate values which cannot be directly measured and to predict physiological changes induced by exercise and medical or surgical therapies.