The physiologic and toxic effects of oxygen at increased pressures during diving and decompression operations are reviewed. The physiologic effects include effects on arterial blood oxygen content that increases with oxygen pressure. The carbon-dioxide carrying capacity of hemoglobin decreases with increasing oxygen pressure causing increased quantities of carbon-dioxide dissolved in the tissues and higher tissue acidity. Increased oxygen pressure causes vasoconstriction and reduces cardiac output, which can reduce removal of dissolved gas from the tissues during decompression. Increased oxygen pressure has transitory effects on ventilation rates, causes an increased capacity for heavy exercise, decreases blood hemoglobin concentration, increases production of electron transport chain components, peroxides, and other oxidation products, and increases the diffusion gradient for elimination of inert gas from body tissues, thus shortening decompression times. Toxic effects of high oxygen pressures depend on duration and partial pressure. Continuous exposure at 0.6 technical atmospheres causes biochemical, morphologic, and functional changes in the lung. At 1 technical atmosphere of oxygen, continued exposure causes fluid accumulation and hemorrhage in the lung causing severe impairment of oxygen transfer into the blood. Neurological effects include tremor and convulsions. Repeated exposure to toxic oxygen pressure can produce cumulative effects in animal models. The mechanisms of oxygen toxicity are reviewed. Individual sensitivity to oxygen toxicity is highly variable and many factors modify oxygen toxicity including age, temperature, hormones, and periodic brief interruptions during oxygen exposure. The authors recommend research on the mechanisms of oxygen toxicity, toxic effects on organ systems and functions other than on lungs and brain, detailed analysis of pulmonary oxygen poisoning, establishing optimal intermittency schedules at different oxygen pressures to reduce acute oxygen poisoning in humans, and development of oxygen therapy in decompression accidents.