Carbon monoxide (CO) is the leading cause of death by poisoning in developed countries. In 2006, 23 deaths and more than 130 hospitalisations related to CO poisoning have been reported in Switzerland. Impairment in oxygen transport through competitive antagonist action on hemoglobin is well known. Other mechanisms, in particular CO induced mitochondrial dysfunction, participate in CO toxicity. CO intoxications carry a high risk of immediate mortality and long-term neuropsychiatric or cardiac sequelae which justify the need of early diagnosis and treatment. Administration of supplemental oxygen is the cornerstone of treatment of CO poisoning. Hyperbaric oxygen therapy must be considered for the treatment of severe intoxications.