Hyperglycemia is associated with increased mortality and morbidity in critically ill patients. Surgical patients commonly develop hyperglycemia related to the hypermetabolic stress response, which increases glucose production and causes insulin resistance. Although hyperglycemia is associated with worse outcomes, the treatment of hyperglycemia with insulin infusions has not provided consistent benefits. Despite early results, which suggested decreased mortality and other advantages of "tight" glucose control, later investigations found either no benefit or increased mortality when hyperglycemia was aggressively treated with insulin. Because of these conflicting data, the optimal glucose concentration to improve outcomes in critically ill patients is unknown. There is agreement, however, that hypoglycemia is an undesirable complication of intensive insulin therapy and should be avoided. In addition, the risk of increased glucose variability should be recognized, because of the associated increased risk for worse outcomes. Patients with diabetes mellitus experience chronic hyperglycemia and often require more intensive perioperative glucose management. When diabetic patients are evaluated before surgery, appropriate management of oral hypoglycemic agents is necessary as several of these agents warrant special consideration. Current recommendations for perioperative glucose management from national societies are varied, but, most suggest that tight glucose control may not be beneficial, while mild hyperglycemia appears to be well-tolerated.