Rates of obesity among adults and children in the U.S. are soaring, with potentially huge private and social costs. Increasing attention is being paid to agricultural policies as both the culprits through their perceived roles in reducing the relative prices of energy-dense foods, and as the potential saviors through their perceived ability to do the opposite. However, the effects of agricultural policies on human nutrition and obesity are not well understood. This paper considers (1) trends in agricultural commodity prices, and the contributions of commodity policies and agricultural R&D policies to those trends, (2) the links between changes in commodity prices and changes in food prices; and (3) the implications of price-induced changes in food characteristics for human nutrition outcomes. Preliminary results suggest that agricultural R&D has led to dramatic decreases in costs of production and to consequent long-term declines in commodity prices, but the links between commodity price declines and food prices are less clear and are conditioned by the changing structure of food markets. Commodity-specific trade policy has clearly put upward pressure on the domestic prices of several major food commodities, but the consumer prices for most of these foods have fallen nonetheless. Changes in relative prices of 'healthy' versus 'unhealthy' foods are difficult to establish empirically, but even if 'healthy' foods are becoming relatively more expensive, food prices in general play only a small role in determining food consumption; hence, policies aiming to reduce obesity through changes in relative food prices may prove ineffective or inefficient.