Digestibility of protein in traditional diets from developing countries such as India, Guatemala, and Brazil is considerably lower compared to that of protein in typical North American diets (54-78 versus 88-94%). The presence of less digestible protein fractions, high levels of insoluble fiber, and high concentrations of antinutritional factors in the diets of developing countries, which are based on less refined cereals and grain legumes as major sources of protein, are responsible for poor digestibility of protein. The effects of the presence of some of the important antinutritional factors on protein and amino digestibilities of food and feed products are reviewed in this chapter. Food and feed products may contain a number of antinutritional factors that may adversely affect protein digestibility and amino acid availability. Antinutritional factors may occur naturally, such as glucosinolates in mustard and rapeseed protein products, trypsin inhibitors and hemagglutinins in legumes, tannins in legumes and cereals, phytates in cereals and oilseeds, and gossypol in cottonseed protein products. Antinutritional factors may also be formed during heat/alkaline processing of protein products, yielding Maillard compounds, oxidized forms of sulfur amino acids, D-amino acids, and lysinoalanine (LAL, an unnatural amino acid derivative). The presence of high levels of dietary trypsin inhibitors from soybeans, kidney beans, or other grain legumes can cause substantial reductions in protein and amino acid digestibilities (up to 50%) in rats and pigs. Similarly, the presence of high levels of tannins in cereals, such as sorghum, and grain legumes, such as fababean (Vicia faba L.), can result in significantly reduced protein and amino acid digestibilities (up to 23%) in rats, poultry, and pigs. Studies involving phytase supplementation of production rations for swine or poultry have provided indirect evidence that normally encountered levels of phytates in cereals and legumes can reduce protein and amino acid digestibilities by up to 10%. D-amino acids and LAL formed during alkaline/heat treatment of proteins such as casein, lactalbumin, soy protein isolate, or wheat proteins are poorly digestible (less than 40%), and their presence can reduce protein digestibility by up to 28% in rats and pigs. A comparison of the protein digestibility determination in young (5-week) versus old (20-month) rats suggests greater susceptibility to the adverse effects of antinutritional factors in old rats than in young rats. Therefore, the inclusion of protein digestibility data obtained with young rats, as the recommended animal model, in the calculation of PDCAAS (Protein Digestibility-Corrected Amino Acid Score) may overestimate protein digestibility and quality of products, especially those containing antinutritional factors, for the elderly. For products specifically intended for the elderly, protein digestibility should be determined using more mature rats.