The purpose of investigating pulmonary gas exchange and pulmonary circulation is detection of functional abnormalities of the cardiorespiratory system. Other than clinical features, a number of specialized methods used today are of high accuracy and therefore serve to recognize early disorders and to quantify functional impairment. In clinical practice arterial blood gas analysis and the measurement of diffusing capacity are sensitive screening tests for patients with acute and chronic respiratory failure and thus have important diagnostic and therapeutic implications (e.g. indication for oxygen therapy, mechanical ventilation, chest physiotherapy, specific drug regimen etc.). Spiroergometry and right heart catheterization, as the more invasive methods, are useful tests in evaluating overall cardiopulmonary function to detect limiting factors in physical performance (e.g. thoracic surgery, occupational diseases, sport medicine).