This article reviews the current body of knowledge regarding lactic acidosis in critically ill patients. The classification of disordered lactate metabolism and its pathogenesis are examined. The utility of lactate as a metabolic monitor of shock is examined and current therapeutic strategies in the treatment of patients suffering from lactic acidosis are extensively reviewed. The paper is designed to integrate basic concepts with a current approach to lactate in critical illness that the clinician can use at the bedside. Comprehensive review of the available, basic science, medical, surgical, and critical care literature. The severity of lactic acidosis in critically ill patients correlates with overall oxygen debt and survival. Lactate determinations may be useful as an ongoing monitor of perfusion as resuscitation proceeds. Therapy of critically ill patients with lactic acidosis is designed to maximize oxygen delivery in order to reduce tissue hypoxia by increasing cardiac index, while maintaining hemoglobin concentration. Buffering agents have not been shown to materially affect outcome from lactic acidosis caused by shock. The benefits of other specific therapies designed to reduce the severity of lactic acidosis remain unproven.