Fat provides essential fatty acids and a concentrated source of energy for dairy cows. The energetic efficiency of milk production is therefore also improved. Of the numerous fat sources that are available, oilseeds, especially cottonseed, probably offer the best opportunity of supplementing fat in dairy diets. It also seems improbable that the gossypol content of whole cottonseed can have detrimental effects if fed at moderate levels and in well-balanced diets. The heat treatment of oilseeds (cottonseed and soybeans) increased milk production in most studies and milk fat in several. The negative effects of fat on rumen microbes can be eliminated to a ‘certain extent by supplementing with extra calcium and magnesium. Although the effect of niacin supplementation has not been consistent in alleviating the negative effect of fats on milk protein it is nevertheless recommended, especially for high-producing cows during early lactation. If fats are to be added above 5% of ration DM, which is the maximum for optimal rumen metabolism, they must be rumen-inert. High melting point fats or calcium salts of long chain fatty acids can be used for additional fat supplementation. However, more consistent results can probably be expected from the use of calcium salts.