With the renewed awareness of blood lactate as an indicator of circulatory impairment, there has been much interest in the use of lactate measurements to determine the overall state of oxygenation of patients in critical care. This review begins by covering the areas of lactate homeostasis and biochemistry, both of which are essential for fully understanding the interpretation of lactate measurements. Then, the clinical interpretation of lactate measurements includes sections on the causes and treatments of hemodynamic deficits leading to lactic acidosis, the classification of lactate abnormalities, and the use of lactate measurements in critical care monitoring, including surgery. Both the principles and the latest developments in lactate methodology are covered, including the new whole blood analyzers. This review concludes with reference intervals and guidelines to the interpretation of results.