The concept of hypothermia as a protective strategy for various kinds of brain injury is far from new. Multiple mechanisms have been implicated including reduction in neuronal apoptosis, inhibition of excitatory processes caused by ischaemia and reperfusion, alterations in intracellular cation concentrations due to ion pump dysfunction, suppression of inflammatory cytokines, reduction of free radical production and reduction of cerebral oedema. Despite support from animal studies, convincing clinical evidence was lacking until recently. Two high quality clinical trials now support the use of hypothermia following cardiac arrest, but a number of issues remain. The main limitation of both trials is their highly selective entry and exclusion criteria, which limit their applicability to the majority of cardiac arrest patients. Questions about the initiation, duration, rewarming rate and the technique for producing hypothermia remain unanswered. There is also concern that side effects of hypothermia have the potential to counteract any potential benefit.