Lactate, indispensable substrate of mammalian intermediary metabolism, allows shuttling of carbons and reducing power between cells and organs at a high turnover rate. Lactate is, therefore, not deleterious, although an increase in its concentration is often a sensitive sign of alteration in energy homeostasis, a rise in it being frequently related to poor prognosis. Such an increase, however, actually signifies an attempt by the body to cope with a new energy status. Hyperlactatemia, therefore, most often represents an adaptive response to an acute energy disorder. Investigation of lactate metabolism at the bedside is limited to the determination of its concentration. Lactate metabolism and acid-base homeostasis are both closely linked to cellular energy metabolism, acidosis being potentially a cause or a consequence of cellular energy deficit.