In the early 1900’s, fat supplements were recognized for enhancing milk fat. A hundred years later, the opposite view prevailed causing major shifts in the types of fats fed to dairy cows to avoid a decline in milk fat. The evolution of this change had more to do with how cows were fed than it did with changes in fat sources. This presentation will review where we are today in recognizing how nature of the basal diet fed to dairy cows is a primary determinant of how a given amount and source of an individual fatty acid will affect milk fat.