Hyperbaric oxygen therapy is defined by the Undersea and Hyperbaric Medical Society as a treatment in which a patient intermittently breathes 100% oxygen while the treatment chamber is pressurized to a pressure greater than atmospheric pressure at sea level. Units of pressure are most commonly expressed as atmosphere absolute although kPa are increasingly being used. Typically, treatments involve pressurization to between two and six ATA, for periods between two and seven hours. Hyperbaric oxygen therapy is a therapy with many complex mechanisms of action. Far from being a therapy in search of a disease, there is now improved understanding of why HBOT may be of benefit in so many apparently disparate disease processes. HBOT should be considered for those indications where there is reasonable pathophysiologic rationale or evidence for use. The UHMS list of indications provides a guideline. As improved data emerges, it is possible that HBOT may prove to be of benefit for some conditions that are currently undergoing trials such as traumatic brain injury, septic shock and burns.