Acute respiratory distress syndrome (ARDS) is a rapidly progressive, non-cardiogenic pulmonary edema characterized by diffuse alveolar damage, refractory hypoxemia, and bilateral pulmonary infiltrates. The pathophysiology involves three phases: exudative (days 1-7) with damage to the alveolar-capillary membrane causing protein-rich fluid to flood the alveoli; proliferative (days 7-21) with type II pneumocyte proliferation and early fibrosis; and fibrotic (after day 21) with potential irreversible scarring. ARDS is defined by the Berlin criteria: acute onset within 1 week, bilateral opacities not fully explained by effusions, respiratory failure not fully explained by cardiac failure, and PaO2/FiO2 ratio classification (mild 200-300, moderate 100-200, severe <100). Common causes include sepsis, pneumonia, aspiration, pancreatitis, massive transfusion, and trauma. Lung-protective ventilation with low tidal volumes (6 mL/kg IBW) and plateau pressures <30 cmH2O is the cornerstone of management.