Several by-product feeds are relatively high in crude protein and exhibit relatively low ruminal degradability, which make them desirable proteinaceous feeds for dairy cows. Therefore, by-product feeds have been and will continue to be important feeds for dairy cows. Factors are discussed that affect ruminal degradability of protein in distillers grains, distillers grains with solubles, brewers grains, corn gluten feed, corn gluten meal, meat meal, meat and bone meal, blood meal, and fish meal, and the potential of these feeds to provide supplemental amino acids needed by lactating dairy cows. The importance of maximizing synthesis of microbial protein and digestion of organic matter in the rumen is emphasized in relation to total amino acid passage to the small intestine. For these feeds to be used most successfully, they must be available from a dependable source at an economical cost and should supply amino acids that complement other amino acids passing to the small intestine. Benefits that should be realized from the successful use of by-product feeds include increased milk production from feeding proteins that have greater ruminal escape potentials and a reduced cost per unit of milk produced because of decreased use of expensive supplemental protein.