Mea'iurements of abnormalities in the interrelationship of pulmonary ventilation and perfusion have become common in oxygen therapy. The techniques by which these measurements are now made and their value in guiding oxygen therapy are discussed. Each phase of oxygen transport to the tissues can be infiuenced by properly regulated oxygen therapy. Oxygen is a poison when given in too high a dose. Pulmonary oxygen toxicity occurs at tensions well below one atmosphere, and at higher tensions almost every cell in the body is affected. The results of hyperbaric oxygen therapy are frequently questionable, and the physiologic rationale often obscure; however, hyperbaric treatment, when properly regulated, will probably continue to have therapeutic application. Likely area‐i of usefulness for hyperbaric medicine are outlined.