The pulmonary vessels reflect in their structure the stresses to which they are subjected. Over the years a variety of techniques have been applied to the study of the pulmonary circulation to acquire a greater understanding of the alterations occurring in various diseases. Congenital heart disease has been of particular interest since it may impose unusual hemodynamic stresses upon the vessels of the lungs. For over two decades we have had the opportunity, first at Yale-New Haven Hospital, New Haven, Connecticut and then at University Hospital, San Diego, California, to study the pulmonary and bronchial circulations in patients with congenital heart disease. Much has been learned from the application of new anatomic tech­ niques to such patients. The clinical correlations, it is to be hoped, will help to bridge the gulf between anatomic and physiologic data. These observa­ tions have been recorded with the thought that pathology often illuminates normal anatomy, and both contribute to the understanding of physiology. Incursions into the several realms should perhaps not be considered trespass but rather an attempt to glimpse more of one country. The sources of help for a book of this scope are bound to be many and varied. My colleague Dr. Averill A. Liebow introduced me to pathology, and through the years of this study we developed a deep respect for each other as scientific colleagues and friends. Dr.