This document discusses acute respiratory distress syndrome (ARDS). It defines ARDS as severe acute lung injury involving diffuse alveolar damage and increased microvascular permeability. It then covers the criteria for diagnosing ARDS, mechanisms of lung injury in ARDS, common direct and indirect causes of ARDS, stages of ARDS, predictors of outcome, findings from the ARDS Network study on low tidal volume ventilation, and treatment strategies for ARDS including ventilator settings and alternative strategies when initial measures fail.