The glycemic index (GI) has proven to be a useful nutritional concept, providing new insights into the relationship between foods and chronic disease. Observational studies suggest that diets with a high glycemic load (GI × carbohydrate content) are independently associated with increased risk of type 2 diabetes and cardiovascular disease. Postprandial hyperglycemia plays a direct pathogenic role in the disease process. Lower glucose and insulin levels are associated with improved risk profile, including high-density lipoprotein cholesterol, glycosylated proteins, oxidative status, hemostatic variables, and endothelial function. Limited evidence suggests that a low-GI diet may also protect against obesity, colon cancer, and breast cancer. Diets with a high glycemic load may affect health differently in insulin-resistant and insulin-sensitive individuals. Improvements in postprandial hyperglycemia can be brought about by manipulating either the type (i.e., GI) or amount of dietary carbohydrate, or both; at present, the GI appears to be more effective.