Acute respiratory distress syndrome (ARDS) is a clinical syndrome characterized by severe respiratory failure, hypoxemia, and diffuse pulmonary infiltrates. ARDS can be caused by direct or indirect lung injury from many underlying medical and surgical disorders. ARDS is defined by three categories - mild, moderate, and severe - based on hypoxemia levels which correlate with mortality risk. The pathophysiology of ARDS involves three phases: an initial exudative phase with pulmonary edema, followed by a proliferative phase of repair, and potentially a final fibrotic phase with scar tissue formation if lung injury persists.