In all manner of acute severe illnesses, oxygen therapy has long been consider as, at worst, harmless, and at best, simple, cheap and highly efficacious. However, there is a large body of accumulating evidence to suggest that hyperoxia is harmful and that mild hypoxaemia may, in fact, be beneficial [1]. At a cellular level, susceptibility to oxygen toxicity appears to be greatest during early reperfusion following ischaemia. Indeed, while our tissues and cells have extensive adaptive mechanisms to hypoxaemia (not least in stimulating local increases in perfusion), they have limited protection from, or adaptation to, hyperoxia.In all but neonates, hyperoxia is the standard of care both during advanced cardiopulmonary resuscitation (CPR) and in the immediate period (minutes to hours) following the return of spontaneous circulation (RoSC). Over recent years, this dogma has started to be questioned in light of emerging, though somewhat equivocal, evidence from both animal models [2] and re ...