Diabetes mellitus is a frequent complication of Cushing syndrome (CS) which is caused by chronic exposure to glucocorticoid excess, either endogenous or exogenous, and that is characterized by several clinical symptoms such as central obesity, purple striae, proximal muscle weakness, acne, hirsutism and neuropsychological disturbances. Diabetes occurs as a consequence of an insulin-resistant state together with impaired insulin secretion which are induced by glucocorticoid excess. The management of patients with CS and diabetes mellitus includes the treatment of hyperglycemia and, when possible, the correction of glucocorticoid excess. This review focuses on the disorders of glucose metabolism in patients exposed to glucocorticoid excess, addressing both the pathophysiological aspects and the clinical and therapeutic implications.