[Plethysmography. The physiological basis of the test and of the various techniques used]. Plethysmography is the evaluation of variations in volumes and pressures of the thorax and pulmonary circulation during breathing. The techniques of examination can be classified by two modalities: 1. "Static" or "traditional": it involves the recording of the maximal variation in thoraco-pulmonary volumes, during a fixed period of time, by means of a recording spirometer (breathing frequency: normal rate: 1 to 1.5/min); 2. "Dynamic": it evaluates, by plethysmography, the pressure variations during tidal inspiration and expiration, recorded by means of a body box (amplitude-time of cycle: 20 to 60 msec, average 40 msec) recording lung pressures in front of the nostril and/or the upper lobe of the lung. This technique is applicable to all pulmonary obstructions (emphysema, thoracic cage tumors) which alter the expiratory flow-volume curve. The dynamic recording technique is more useful in establishing the diagnosis of a particular lesion, but the sensitivity of this technique is lower than the static recording. There is no significant difference between the results obtained with the three techniques described for measuring the volume of tidal breath and for the determination of maximal expiratory volume, whereas the determination of maximal inspiratory volume using the dynamic recording technique gives the best results. In the detection of a right-left shift in the diaphragm, the dynamic recording technique is more sensitive than the static recording technique. In practice, plethysmography is an indispensable method which enables the practitioner to determine the site of lesions in thoracic and thoracic cage tumors. It provides important information concerning the mechanics of respiration in certain respiratory or thoracic cage disorders and when respiratory muscular involvement is present. As a result of measuring the work of breathing, thoracic and pulmonary pletysmography can be useful in assisting surgical techniques (plasties, laryngectomy, chest wall resections). A better evaluation of the work of breathing, lung and thoracic cage resistances, can permit a more rational choice of ventilation apparatus (or no ventilation at all) and help in the management of the patient. Finally, the knowledge of the characteristics of pulmonary volumes and volumes and pressures of the thorax during tidal breathing is an excellent diagnostic and a highly reliable guide for many lung function tests.