This thesis will describe the author's early experience in the study of congenital heart disease using two-dimensional echocardiography, which was, at the start of the work, a new diagnostic technique. A brief description will be given of the equipment, a real-time two-dimensional ultrasonic sector scanner developed by a team of electronic and mechanical engineers in the Department of Clinical Physics and Bio-engineering, Glasgow. The limitations of the original design will be outlined and the subsequent modifications which improved the image quality and were necessary to provide satisfactory studies from small infants will be described. Application of the technique first required that the normal two-dimensional echocardiographic anatomy be established; techniques which were developed for this, scanning plane's which were considered to give a full echocardiographic display of the anatomy of the heart, and the appearances in a group of normal infants and children will be described. A technique of examination having been established, infants and children with the following types of complex congenital heart disease were studied: transposition of the great arteries, corrected transposition of the great arteries, double outlet right ventricle, persistent truncus arteriosus, tetralogy of Fallot, pulmonary atresia with and without a ventricular septal defect, aortic atresia, and total anomalous pulmonary venous drainage. For each condition the echocardiographic appearances will be presented, their significance discussed, and, where possible, diagnostic criteria will be described. The findings in patients with atrioventricular canal malformations will be considered separately; echocardiographic features considered to identify the partial atrioventricular canal and the type A and the type C complete atrioventricular canal with standard and contrast echocardiography will be reported. The findings in the initial studies were applied in the study of a consecutive series of 64 cyanosed infants examined before cardiac catheterisation and angiocardiography; the results will be assessed and the diagnostic accuracy of two-dimensional echocardiography evaluated. Finally, the results described in this thesis and by other workers will be applied to suggest an approach to the diagnosis of congenital heart disease with two-dimensional echocardiography based on the principles of sequential chamber localisation, and to outline the present place of two-dimensional echocardiography in the investigation of patients with congenital heart disease.