Since the adult respiratory distress syndrome was first described substantial progress has been made in understanding the pathogenesis of this complex syndrome. This review summarizes our current understanding of the pathophysiology of what is now termed the acute respiratory distress syndrome (ARDS) and its less severe form acute lung injury (ALI), with an emphasis on cellular and molecular mechanisms of injury that may represent potential therapeutic targets. Although it is difficult to synthesize all of these abnormalities into a single, unified, pathogenetic pathway, a theme that emerges repeatedly is that of imbalance, be it between pro- and anti-inflammatory cytokines, oxidants and antioxidants, procoagulants and anticoagulants, neutrophil recruitment and activation and mechanisms of neutrophil clearance, or proteases and protease inhibitors. Future therapies aimed at restoring the overall balance of cytokines, oxidants, coagulants, and proteases may ultimately be successful where therapies that target individual cytokines or other mediators have not.