As a systemic disorder, diabetes affects both the central and peripheral nervous systems. Virtually every aspect of the nervous system and its supporting structures, from the cerebral cortex to the effector organs, may be involved with the complications of diabetes. These involvements are caused by the direct effect of hyperglycemia or hypoglycemia and attempts at their control; hyperglycemia-induced metabolic derangements; neurochemical alterations; serum lipid changes; and vascular, coagulation, and thrombotic abnormalities. Other neurologic conditions may be associated with, but not caused by, diabetes, including Friederich's ataxia and myotonic muscular dystrophy, and, in some cases, similar immunopathogenetic mechanisms may be shared (e.g., stiff-man syndrome and type I diabetes.) This article deals specifically with the complications of diabetes on the nervous system.