For the precise formulation of the protein (amino acid) component of diets for monogastric animals such as the pig and chicken, it is of vital importance to have information regarding the nutritional quality of the protein sources available. Determinations of the nutritional availability of the amino acids must be carried out with ileal digesta rather than with feces. For feedstuffs that have not undergone processing (with heat and pressure, for example), true ileal amino acid digestibility coeffcients are the most appropriate for use in practical diet formulation. However, for feedstuffs that have undergone processing, the use of conventional methods to determine true ileal amino acid digestibility will overestimate the availability of amino acids such as lysine. In these cases, different methods must be used to determine the availability of lysine, such as the O-methylisourea or true reactive lysine bioassay method. The objective is to present a critical evaluation of the appropriateness of the use of the different methodologies currently available to determine the availability of amino acids in protein sources, both processed and non-processed, for the monogastric animals.