Stress-induced hyperglycemia is a common problem in patients admitted to the ICU, even when glucose homeostasis has previously been normal. Hyperglycemia is a near-universal finding in diabetic persons suffering catabolic illnesses and may worsen prognosis, although this possibility is controversial. The presence of hyperglycemia is associated with an increased risk of infectious complications in surgical patients, and indirect evidence indicates that maintenance of euglycemia can reduce the risk of infection. The causes of stress hyperglycemia include the presence of excessive counterregulatory hormones (glucagon, growth hormone, catecholamine, and glucocorticoid, either endogenous or exogenous), high circulating or tissue levels of cytokine (in particular tumor necrosis factor-⍺ [TN⍺] and interleukin-1). This metabolic milieu results in failure of insulin to suppress hepatic gluconeogenesis despite hyperglycemia; in addition, insulin-mediated glucose uptake into skeletal muscle is impaired. Patients given excessive nutritional support, especially by the intravenous route, are particularly likely to encounter hyperglycemia. Insulin remains the obvious treatment for hyperglycemia, although evidence documenting the clinical benefit of aggressive insulin therapy in the ICU is sparse.