Clinically recognized disorders of glucose metabolism include impaired fasting glucose, impaired glucose tolerance (both termed prediabetes), and diabetes mellitus. Type 2 diabetes mellitus affects 6% to 13% of adults in the United States. Among patients with recent stroke, 70% will have known diabetes, occult diabetes (detectable on an oral glucose tolerance test), or prediabetes. Type 2 diabetes mellitus is associated with a two- to six-fold increased risk for first or recurrent ischemic stroke. The mechanisms for the association are myriad and include the effects of hyperglycemia on vascular tissues and coagulation, and aberrations in blood pressure regulation, lipid metabolism, endothelial function, vascular inflammation, lipid metabolism, smooth muscle cell proliferation, and fibrinolysis. The most effective strategies to prevent stroke among people with diabetes include blood pressure control, antiplatelet therapy, and statin therapy. Tight glycemic control is recommended to prevent microvascular disease, but the effect on macrovascular disease, including stroke, has not been proven. Target blood pressure should be less than 130/80. Antiplatelet therapy may be accomplished with 81 to 325 mg of aspirin daily or 75 mg of clopidogrel daily. Statins should be given in dosages effective to reduce low-density lipoprotein cholesterol to less than 100 mg/dL. For glycemic control, first line therapy for most patients is metformin, starting at 500 mg daily. With time, most patients will need two or three oral medications from different classes and many eventually will require insulin therapy. Prevention of diabetes may be best accomplished by identifying those at risk and modifying diet, weight, and exercise habits. Screening for prediabetes and diabetes is appropriate for men and women older than 45 years and all individuals with vascular disease. Insulin resistance and impaired insulin secretion is the major underlying defect in type 2 diabetes mellitus. It also affects 50% of nondiabetic subjects with a recent ischemic stroke. Emerging evidence has linked insulin resistance to the pathophysiologic derangements in type 2 diabetes mellitus that accelerate atherosclerosis. Treatment of insulin resistance with weight loss, exercise, or medication can correct these derangements, and represents a promising approach to stroke prevention.