We are slowly getting to grips with the mechanisms underlying shock. Many shock states involve a decreased supply of oxygen to the tissues, e.g. following a major haemorrhage. The problem may continue – or even arise from – a decrease in utilization of oxygen at the cellular level. The oxygen is predominantly used by the cell's mitochondria to generate ATP. This energy failure appears to underlie multiple organ failure in the critically ill patient and offers new routes for therapeutic intervention.