Amino acid nutrition of monogastrics has been investigated using individual amino acids since the discovery of threonine by W. C. Rose in 1935. With commercial availability of synthetic methionine and crystalline lysine in the early 1960s, and because these two amino acids are typically first limiting in poultry and swine, there has been a plethora of research estimating their required needs. As additional crystalline amino acids became commercially available, tryptophan and threonine, and isoleucine and valine, research on their estimated requirements and ratios relative to lysine has expanded our understanding on how to formulate diets to minimize amino acid excesses while meeting the nutritional needs of the animal. Because of the increased availability of crystalline amino acids and the continual need to improve the utilization of nutrients to reduce the impact of livestock production on the environment, there is always a need to more fully understand amino acid nutrition of non-ruminants. In addition, characterization of ingredients or diets (total, apparent, standardized, or true, etc.), and method of data analysis, still present a difficult task of summarizing published data into a concise data set for ultimate use in feed formulation. Lastly, determination of nutrient requirement estimates in older or heavier animals is costly and inherently variable such that substantial progress in refining their nutrient needs has been difficult. However, progress in understanding nutrient needs of these animals is vitally important since consumption of feed is greatest, and conversion efficiency into edible product is lowest, at these heavier weights.