This chapter discusses various aspects of decompression, radiation, and altitude sickness. Decompression sickness is a disorder in which nitrogen dissolved in the blood and tissues by high pressure forms bubbles, as pressure decreases. Pulmonary decompression sickness presents with burning pleuritic substernal pain, cough, and dyspnea. The extent of damage due to radiation exposure depends on the quantity of radiation delivered to the body, the dose rate, the organs exposed, the type of, the duration of exposure and the energy transfer from the radioactive wave to the exposed tissue. Radiation sickness occurs when the X-ray therapy is given over the abdomen, less often with thorax and rarely, when given on the extremities. Five manifestations of altitude illness are acute mountain sickness, high-altitude pulmonary edema, high-altitude encephalopathy, subacute mountain sickness, and chronic mountain sickness. Voluntary periodic hyperventilation may relieve symptoms and definitive treatment is immediate descent, which is essential, if reduced consciousness, ataxia, or pulmonary edema occurs.