The modern era in cardiopulmonary medicine began in the 1940s, when Cournand and Richards pioneered right-heart catheterization. Until that time, no direct measurement of central vascular pressure had been performed in humans. Right-heart catheterization ignited an explosion of insights into function and dysfunction of the pulmonary circulation, cardiac performance, ventilation-perfusion relationships, lung-heart interactions, valvular function, and congenital heart disease. It marked the beginnings of angiocardiography with its diagnostic implications for diseases of the left heart and peripheral circulation. Pulmonary hypertension was discovered to be the consequence of a large variety of diseases that either raised pressure downstream of the pulmonary capillaries, induced vasoconstriction, increased blood flow to the lung, or obstructed the pulmonary vessels, either by embolism or in situ fibrosis. Hypoxic vasoconstriction was found to be a major cause of acute and chronic pulmonary hypertension, and surprising vasoreactivity of the pulmonary vascular bed was discovered to be present in many cases of severe pulmonary hypertension, initially in mitral stenosis. Diseases as disparate as scleroderma, cystic fibrosis, kyphoscoliosis, sleep apnea, and sickle cell disease were found to have shared consequences in the pulmonary circulation. Some of the achievements of Cournand and Richards and their scientific descendents are discussed in this article, including success in the diagnosis and treatment of idiopathic pulmonary arterial hypertension, chronic thromboembolic pulmonary hypertension, and management of hypoxic pulmonary hypertension.