The current feed protein evaluation systems taking into account the amount of amino acids absorbed from the small intestine are critically reviewed. Especially the differences in the concepts of estimating protein values and limitations of techniques used to estimate parameter values are discussed. Validation of different approaches to estimate the microbial component of metabolizable protein using data from production experiments was conducted. The review is focused on empirical models based on factorial approaches. In the future, the mechanistic models, which can better take into account interactions in the digestive tract and tissue metabolism, should be developed.