Cardiovascular anomalies are the most common birth defects in man, accounting for 0.5–1% of live births. They are typically classified according to the affected segment of the heart. The complexity of an anomaly, its relationship to other cardiac structures, and its physiologic consequences, determine its clinical significance. Ventricular and atrial septal defects are the two anomalies most commonly seen and can produce a spectrum of problems, including congestive heart failure and pulmonary hypertension (1 ). Abnormal atrioventricular valves, such as found in tricuspid atresia or congenital mitral stenosis, are associated with underdevelopment of either the right or left ventricle, respectively, and have more severe clinical implications. Maldevelopment of either the aorta or the pulmonary artery leads to conditions such as tetralogy of Fallot, double-outlet right ventricle, and transposition of the great arteries. All of these congenital anomalies require surgical intervention for long-term survival. The events involved in normal cardiac development are described in Chapter 5 in Vol. II in this series. This chapter addresses a method that enhances the study of cardiac dysmorphogenesis.