Glycine, proline, and hydroxyproline (Hyp) contribute to 57% of total amino acids (AAs) in collagen, which accounts for one-third of proteins in animals. As the most abundant protein in the body, collagen is essential to maintain the normal structure and strength of connective tissue, such as bones, skin, cartilage, and blood vessels. Mammals, birds, and fish can synthesize: (1) glycine from threonine, serine, choline, and Hyp; (2) proline from arginine; and (3) Hyp from proline residues in collagen, in a cell- and tissue-specific manner. In addition, livestock (e.g., pigs, cattle, and sheep) produces proline from glutamine and glutamate in the small intestine, but this pathway is absent from birds and possibly most fish species. Results of the recent studies indicate that endogenous synthesis of glycine, proline, and Hyp is inadequate for maximal growth, collagen production, or feed efficiency in pigs, chickens, and fish. Although glycine, proline and Hyp, and gelatin can be used as feed additives in animal diets, these ingredients except for glycine are relatively expensive, which precludes their inclusion in practical rations. Alternatively, hydrolyzed feather meal (HFM), which contains 9% glycine, 5% Hyp, and 12% proline, holds great promise as a low cost but abundant dietary source of glycine, Hyp, and proline for ruminants and nonruminants. Because HFM is deficient in most AAs, future research efforts should be directed at improving the bioavailability of its AAs and the balance of AAs in HFM-supplemented diets. Finally, HFM may be used as a feed additive to prevent or ameliorate connective tissue disorders in domestic and aquatic animals.