Pulmonary hypertension is a complex disease that causes significant morbidity and mortality and is clinically characterized by an increase in pulmonary vascular resistance. The histopathology is marked by vascular proliferation/fibrosis, remodeling, and vessel obstruction. In this book, the authors present current research in the study of the pathogenesis, diagnosis and treatment of pulmonary hypertension. Topics discussed include the pathophysiology of Eisenmenger Syndrome (ES) and an evaluation of differences between ES and pulmonary arterial hypertension; pulmonary endarterectomy as an effective treatment for chronic thromboembolic pulmonary hypertension; liver transplantation and pulmonary hypertension and the possible etiologies of pulmonary hypertension in Down Syndrome children. © 2012 by Nova Science Publishers, Inc. All rights reserved.