The deadly effect of carbon monoxide was known as long ago as Greek and Roman times, when the gas was used for executions1. In 1857 Claude Bernard postulated that its noxious effect was caused by reversible displacement of oxygen from haemoglobin to form carboxyhaemoglobin2. In 1926 it became apparent that hypoxia was caused not only by deficient oxygen transport but also by poor tissue uptake. Warberg used yeast cultures to show that cellular uptake of oxygen was inhibited by exposure to a large amount of carbon monoxide3. Carbon monoxide is known as the silent killer since it has no colour or smell. Each year in Britain about 50 people die and 200 are severely injured by carbon monoxide poisoning4. Some poisonings are caused by self-harm but most are accidental5. It is the commonest cause of accidental poisoning and, according to one estimate, as many as 25000 people in the UK have symptoms due to faulty gas appliances4. In the 1960s and 1970s the conversion from coal gas to carbon-monoxide-free natural gas caused a dramatic reduction in poisoning6. In this review I discuss modern approaches to management and prevention.