INTRODUCTION: The initiation of oxygen therapy for acute or chronic respiratory failure is a common medical intervention, both for hospital in-patients and in out-of-hospital emergency settings. Oxygen therapy is also frequently initiated by paramedics or nurses, without any initial medical prescription, in acutely ill patients. STATE OF THE ART/PERSPECTIVES: It is important to remember that oxygen is a pharmaceutical drug, and its prescription should therefore be considered within treatment guidelines. Two main pathological situations may be encountered: tissue hypoxia and acute/chronic hypoxemia. CONCLUSION: Physicians should be aware of the clinical signs that may indicate the presence of hypoxia and the pathological situations that may lead to hypoxemia. They must also be aware of the potential complications, in particular CO(2) retention in patients with chronic type II respiratory failure as well as the overall indications, and practical issues concerning oxygen therapy.