A significant change in the assessment of dietary protein quality occurred with the introduction of the protein digestibility corrected amino acid score (PDCAAS) which combines traditional chemical scoring of a protein, based on its amino acid (AA) composition relative to requirements, with a biological assessment of AA availability. This paper focuses on several issues relating to determining the availability of AAs in a protein. The impact of using ileal versus faecal, and true versus real digestibility coefficients to assess protein quality is discussed.