Book excerpt: Although diagnosis always begins with a careful history and physical examination and a physician is obligated to consider more than the diseased organ, testing of lung function has become standard practice to confirm the diagnosis, evaluate the severity of respiratory impairment, assess the therapy response and follow-up patients with various cardio-respiratory disorders. Ventilation, diffusion, blood flow and control of breathing are the major components of respiration and one or more of these functional components can be affected by any disorder. Frequently, no single pulmonary function test.