According to Reynolds Kitchen, the difference in appearance between the two sides of aluminum foil is simply a result of manufacturing and serves no real purpose. Both sides of the foil are equally effective at heating your food, so it doesnt matter which side you use when cooking. However, if you are using non-stick foil, you should use the dull side, since only one side has the non-stick coating applied. Reynolds has imprinted the words "NON-STICK SIDE" onto the side with the non-stick coating to make it easy to identify.