Acute respiratory failure, and the need for mechanical ventilation, remains one of the most common reasons for admission to the intensive care unit (ICU). The burden of acute respiratory failure is high in terms of mortality and morbidity as well as the cost of its principal treatment, mechanical ventilation. Very few epidemiologic studies have evaluated the prevalence and outcome of acute respiratory failure and mechanical ventilation in general. Most of the published literature has focused on specific forms of acute respiratory failure, particularly acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). In this chapter, we provide a brief review of the pathophysiology of acute respiratory failure, its definition and classification, and then present the incidence and outcomes of specific forms of acute respiratory failure from epidemiologic studies.