Summary Hyperbaric oxygen therapy is a treatment with a number of different indications. It is managed by different specialities, but most of the hyperbaric facilities are driven by departments of anaesthesiology, and some of the patients are critically ill. This paper deals briefly with modes of administration, mechanisms of action, indications and risks. Two indications, carbon monoxide poisoning and necrotising soft tissue infections, are of special interest for the anaesthesiologist, and are therefore dealt with in little more detail.