This chapter discusses the diagnosis, evaluation and management of acute respiratory distress syndrome (ARDS). Pulmonary edema in ARDS is heterogeneous and leads to atelectatic or consolidated areas of lung interspersed with relatively unaffected regions, creating areas of intrapulmonary shunt, which results in hypoxemia that does not improve with oxygen administration alone. As pulmonary edema accumulates in the initial exudative phase of the disease, patients become dyspneic and demonstrate increased work of breathing. Due to worsening lung compliance, tidal volumes decrease and respiratory rate increases. Patients become progressively hypoxemic due to both worsening V/Q mismatch and shunt physiology. The respiratory failure is not fully explained by cardiac failure or volume overload. If a known risk factor for ARDS is not present, objective assessment such as echocardiography should be obtained to rule out hydrostatic edema. On a chest radiograph, ARDS looks essentially the same as cardiogenic pulmonary edema.