Advancements are discussed for predicting and optimizing microbial protein as a function of rumen-degraded carbohydrates and correcting for amino acids (AA) in true protein. Intestinal digestibility of rumen-undegraded protein (RUP) is being evaluated primarily using in vitro approaches in field situations, but unfortunately few comparisons to in vivo intestinal digestibility are available in dairy cattle. The AA profile of feeds in libraries has been improved, but interactions between feed category and method prevent systematic prediction of differences in rumen degradability and intestinal digestibility of individual AA among feeds. We discuss a factorial approach to essential AA requirements and models predicting how to meet those requirements. Efficiencies of conversion of metabolizable AA into milk protein vary inversely with their respective supplies but also depend on supply of energy. We describe differences among models to estimate protein and AA requirements and to predict milk protein yield, and future research needs are emphasized.