Catheterization is rarely used to learn the basic anatomy of the heart. The diagnostic information necessary for most cardiac surgical procedures in neonates is now obtained noninvasively by echocardiography. Diagnostic cardiac catheterization is used to provide specific data unavailable through echocardiography that are useful in planning management (47). What is the anatomy of the pulmonary arteries and systemic-to-pulmonary collaterals in the patient with tetralogy of Fallot and pulmonary atresia? (see Tetralogy, Fig. 33-22) Is catheter closure of the collaterals possible? What is the anatomy of the coronary arteries in the patient with pulmonary atresia and intact ventricular septum? (see Pulmonary Atresia, Fig. 33-26) What is the anatomy of the coronary arteries with tetralogy of Fallot or transposition in which abnormality is suspected and/or echocardiographic imaging is nondiagnostic? In selected patients with cardiomyopathy, light and electron microscopic analysis of ultrastructural anatomy and biochemical analysis of myocardium obtained by biopsy may provide a diagnosis. In many situations diagnostic catheterization may be safer or more useful after initial palliative surgery, such as in hypoplastic left heart syndrome or following shunt procedures in those with complex intracardiac anomalies and pulmonary atresia.