Stress is an important contributor to pathological conditions in humans. Hormonal changes that occur during acute and chronic stress situations can affect glucose homeostasis in both healthy people and in those with diabetes. Several studies have reported a negative effect of acute stress on maintenance of blood glucose concentrations in patients with type 1 and type 2 diabetes. The effect of stress on glycemic control in people with diabetes may be related to a direct effect of stress hormones on blood glucose levels and an indirect effect of stress on patient behaviors related to diabetes treatment and monitoring and meal and exercise plans. In contrast, there is no clear evidence that stressful life events promote the development of diabetes in children or in adults. Stress hyperglycemia, the development of hyperglycemia during acute illness, represents another interesting connection between the stress system and glucose homeostasis. A large body of evidence supports an association between stress hyperglycemia and increased morbidity and mortality in critically ill patients. Interestingly, there is some evidence supporting a beneficial effect of insulin in reducing morbidity and mortality in patients admitted to intensive care units. Finally, stress can influence the development of type 2 diabetes indirectly by promoting obesity and metabolic syndrome.