Insulin resistance is a metabolic disorder that is increasing worldwide and is associated with some of the most common diseases affecting modern societies including diabetes, hypertension, obesity and coronary heart disease. Although pharmacologic approaches to managing insulin resistance are being advocated by some, public health approaches involving changes in diet and physical activity are attractive because of their lower cost and risk. We briefly summarize some new information on the mechanisms that mediate insulin's many biological actions and examine the effects of dietary carbohydrates on insulin sensitivity. Specifically, we summarize some of the information available on the effects of simple sugars, complex carbohydrates including fiber, slowly digested starch and the general concept of glycemic index. The available data support the idea that consumption of diets high in total carbohydrate does not adversely affect insulin sensitivity compared with high fat diets. Animal data suggest that simple sugars, in particular fructose, have adverse effects on insulin action, but adverse effects have not been shown conclusively in humans. Increased intake of dietary fiber appears to improve insulin action and may protect against the development of diabetes. The effects of diets with high or low glycemic index on insulin action are controversial at this time. For firm conclusions to be reached, future studies must be of reasonable duration, be in defined populations and compare the effects of relevant doses of nutrients on specific endpoints of insulin action.