Hypoxic-ischemic brain injury is a well known consequence of cardiac arrest. Variable injuries can occur with purely hypoxic or histotoxic insults such as asphyxiation and carbon monoxide poisoning. The injury may happen at the time of the insult, but there may also be continued damage after circulation and oxygenation are reestablished. The nature and extent of the damage appears to depend on the severity, time course and duration of the oxygen deprivation and lack of blood supply, as well as on the underlying mechanism. This review describes the pathophysiological and molecular basis of hypoxic ischemic brain injury, and differentiates between the mechanisms of injury by cardiac arrest, pure respiratory arrest, and arrest secondary to cytotoxicity (e.g. carbon monoxide poisoning).