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Hyperbaric oxygen therapy (HBO) is increasingly used in a number of areas of medical practice. It is a unique intervention whose method of action is not well understood. Clinicians may request its use for their patients, but often will not fully understand its mechanisms. It is hoped that this review and discussion of HBO and the literature surrounding its use may be useful to clinicians who are unsure whether their patients will benefit from this exciting intervention. Hyperbaric oxygen therapy is defined by the Undersea and Hyperbaric Medical Society (UHMS) as a treatment in which a patient intermittently breathes 100% oxygen while the treatment chamber is pressurized to a pressure greater than sea level (1 atmosphere absolute, ATA).1 The pressure increase must be systemic, and may be applied in monoplace (single person) or multiplace chambers. Multiplace chambers are pressurized with air, with oxygen given via face-mask, hood tent or endotracheal tube; while monoplace chambers are pressurized with oxygen. We began by obtaining the most recent UHMS committee report,1 and performed Medline searches (1966 to present), with the search terms ‘hyperbaric’ and ‘oxygen’, combining this basic search with searches for each of the thirteen indications recommended by the UHMS. Using information from these papers, and the resulting references, this paper outlines the history, physiology, current indications for and effects of hyperbaric oxygen therapy. Hyperbaric therapy was first documented in 1662, when Henshaw built the first hyperbaric chamber, or ‘domicilium’.2 Since this time, reports of beneficial effects from increased pressure have increased, and by 1877, chambers were used widely for many conditions, though there was little scientific rationale or evidence. In 1879, the surgical application of hyperbaric therapy in prolonging safe anaesthesia was realized and explored.3 In 1927, Cunningham4 reported improvement in circulatory disorders at sea level … Address correspondence to Dr C.N.A. Bell, Division of Oral & Maxillo-Facial Surgery, Bristol Dental Hospital, Lower Maudlin Street, Bristol BS1 2LY. e-mail: chris.bell{at}bristol.ac.uk