While hyperlactatemia in postoperative cardiac surgery patients was once believed to solely reflect hypoperfusion, either from the accumulated "oxygen debt" during bypass or ongoing inadequate perfusion, our understanding of lactate generation, clearance, and management has evolved. A contemporary understanding of lactate balance is critical to the management of the postoperative patient with hyperlactatemia. In this review, we summarize the current understanding of lactate metabolism in pediatric patients following cardiac surgery and highlight two types of hyperlactatemia: type A, which is secondary to inadequate oxygen delivery and tissue hypoxia, and type B, which in postoperative pediatric cardiac surgery patients largely reflects increased glycolysis driven by the stress response. Both types may coexist; thus, it is imperative that providers first assess the patient for evidence of hypoperfusion. In patients with evidence of adequate perfusion, a type B component is often associated with a concomitant balanced (normal anion gap) metabolic acidosis and hyperglycemia. These patients will benefit from a more nuanced approach to their type B hyperlactatemia, as many will have a benign course and may be managed expectantly.