There is an abundance of information in the literature on apparent ileal amino acid digestibility values in feedstuffs for pigs. In addition to differences in ileal amino acid digestibility values between feedstuffs, there are surprisingly large differences in ileal amino acid digestibility values among different samples of the same feedstuff (in name). In addition to different processing conditions and inherent factors among samples of the same feedstuff, a proportion of the variation can be attributed to different approaches in methodology to determine amino acid digestibility values. It should also be kept in mind that other factors, including the technique used for collection of ileal digesta, may affect, depending on the feedstuff in question, amino acid digestibility values. As far as methodology is concerned, differences in dietary amino acid levels may contribute to the variation in ileal amino acid digestibility values within the same feedstuff. Dietary amino acid levels quadratically affect ileal amino acid digestibility values. In order to remove the effect of dietary amino acid levels, the plateau ileal amino acid digestibility values should be determined. Methods of determination, the direct versus difference versus regression method, can also affect ileal digestibility values within the same feedstuff. In order to estimate this variation, methods of determination specifically suited for different feedstuffs are recommended. As a concluding remark, it is important to take into account the previous considerations when amino acid digestibility values from the literature are compiled for the purpose of reference for diet formulation for pigs.