Analysis of the volume versus time curve during a maximum effort forced expiratory vital capacity maneuver started from total lung capacity (TLC) is by far the most performed test of respiratory mechanics, having spread from the laboratory to the wards and outpatient clinics and gradually into the offices of general practitioners. In particular, forced expiratory volume in 1 second (FEV1) is the best characterized test of respiratorv function; information on changes with age, gender, ethnic group, growth and disease is more developed than for any other test, repeatability is good, and it provides useful information across the whole range from normal to advanced disease.