Main directions of methodical development are characterized and advantages as well as disadvantages are discussed from a critical perspective for the purpose of an introduction into the practice of diet formulation. One of the most important problems is that based on tabulated data we have only limited possibilities to reliably characterize the amino acid availability of actual feed batches. Endogenous secretions of amino acids and the difficulty for quantitative estimations under different dietary conditions are a further point of significance. From this point of view more standardized methods and a better knowledge about variation factors within different batches of one feed protein, including feed treatments, are prerequisites for further successful developments.