Lactate has traditionally been framed as a by-product of anaerobic metabolism and a surrogate for tissue hypoxia. Contemporary physiology recasts lactate as a central fuel, shuttle, and signaling molecule, challenging long-held interpretations in cardiac surgery and intensive care. This narrative review synthesizes evidence across exercise physiology, biochemistry, cardiac anesthesia, and intensive care to reinterpret perioperative hyperlactatemia during cardiopulmonary bypass and after cardiac surgery and highlights emerging opportunities in continuous lactate monitoring.