Oxygen is an essential gas. Oxygen is also a biological medicine. Hyperbaric oxygen therapy is a treatment which is based on the respiration of pure oxygen in a particular pressurised environment (hyperbaric chamber). The pressure allows the diffusion of oxygen into the blood at a concentration which is ten/fifteen/twenty times the normal level. The increase in oxygen in bodily liquids stimulates the synthesis of a gas, nitric oxide (NO), which has a powerful anti-inflammatory effect and promotes the formation of new blood vessels (also through the employment of stem cells) thus accelerating the healing of wounds. Hyperbaric oxygen therapy reactivates metabolic processes which have stopped and is able to help the recovery and obvious improvement of patients suffering from several serious illnesses. Hyperbaric oxygen therapy is a medicine, and as such requires careful dosage, monitoring of its results, and prevention of possible side effects.