The basis for using determined values of endogenous ileal protein and amino acid losses in the protein evaluation of pig feeds is reviewed. The amounts of endogenous protein and amino acids in ileal digesta may be determined by numerous methods. These include N-free diets, regression analysis, enzyme hydrolysed casein (EHC) coupled with ultra-filtration, modifying lysine to homoarginine, isotope dilution techniques and calculations based on the difference between thein vitroandin vivodigestibility of protein and amino acids.