Uncontrolled diabetes increases the risk of postoperative complications including surgical site infection, acute renal failure, and mortality. Further, the stress of surgery and anesthesia increase counter regulatory hormones which contribute to an insulin resistant state and can lead to hyperglycemia, even in patients without known diabetes. Hyperglycemia impairs neutrophil function, and increases inflammatory cytokines and formation of reactive oxygen species. These changes lead to cellular damage and dysfunction of the immune and vascular systems. Recognizing patients at risk for perioperative hyperglycemia during the preoperative assessment provides an opportunity to intervene on glycemic control. Further, attention to glycemic control on the day of surgery and postoperatively with active management of blood glucoses with insulin has been shown to improve outcomes. This article reviews perioperative complications associated with diabetes and hyperglycemia, and provides guidelines for assessment and management of the patient preoperatively, on the day of surgery and postoperatively. It also provides a protocol for treating hypoglycemia in the hospital setting.