The dietary requirement for protein is actually a requirement for the amino acids contained in the dietary protein. The efficiency of protein utilization depends to a large extent on the amino acid composition of the diet. Although a large volume of published amino acid digestibility values for poultry feedstuffs is available, there are still many gaps in our knowledge on digestibility and utilization of amino acids. The major focus of this thesis was to examine several aspects of amino acid digestibility and utilization in broiler chickens. Initial studies examined the performance of two commercial broiler strains fed diets formulated on total or digestible amino acids. The objective of this study was to determine the individual bird response to 4 different diets formulated on: 1) total amino acids; 2) digestible amino acids (book values)using the same ingredient as diet 1; 3) digestible amino acids values determined on the same ingredients as used in diet 1: and 4) digestible amino acids but formulated commercially. The results show that birds given diets formulated on digestible amino acid basis, grew faster, ate more feed and converted it into body weight more efficiently, and had a higher proportion of body protein (P<0.05) than birds given diet formulated on a total amino acid basis. It was also shown that males broiler were significantly faster growing, consumed more feed and had better feed efficiency than do female broilers. There was also a significant (P<0.05)sex x diet interaction for weight gain. Whilst, males always grew faster than females, the difference was much greater for those given the digestible amino acid formulated diets. The studies reported in Chapters 5 and 6 was conducted to determine the digestible methionine, lysine and threonine requirement of broilers during the starter and finisher periods. The minimum digestible methionine requirements to maximize weight gain for starter was similar with grower. Digestible lysine requirements to maximize weight gain represent a decrease 25%, respectively from starter to grower. The lysine digestible requirement for weight gain decreased from 11.4 g/kg diet (0 to 3 wk) to 8.5 g/kg diet (3 to 6 wk). For threonine, the starter threonine level of 6.5 g/kg resulted in the best responses (P<0.01). The study reported in Chapter 7 was to identify the extent of differences in the nutritional value of sorghum available within Australia, in particular the apparent digestibility of amino acids in sorghum samples based upon digesta samples taken from the lower ileum of the chicken. The results of this study suggest that it is possible to estimate digestible amino acid supply from sorghum from the protein content of the grain. Crude protein content (N x 6.25) of sorghum varied from 80.2 to 117.5 g/kg dry matter. In most Australian cereal based diet lysine is first limiting amino acids. In this experimental diet, methionine is the first limiting amino acid content ranged from 1.06 to 1.42%. It followed by lysine, histidine and glycine, respectively. The cereal-based diets fed to poultry contain substantial quantities of phytin,which is poorly digested by poultry. The poor digestive utilization of phytin-bound P by poultry and its consequences on digestibility of protein and amino acids lead to do some more extensive research in this aspect. In this regard, the effects of enzyme supplementation on apparent ileal amino acid digestibility of cereal were studied in Chapter 8. Supplementing wheat fed to 5-wk-old broilers with xylanase alone or a combination of xylanase and phytase improved the ileal digestibility of amino acids. Although there was improvement in sorghum based diet, phytase and xylanase supplementation had no effect on the avaergae of ileal amino acid digestibility. Overall, the results presented in this thesis demonstrate that formulation of poultry diets on a digestible amino acid basis is superior to formulation of diets on a total amino acid basis. This study suggested that genotype and sex differences in performance and body composition should be taken into account when formulating diets to maximize performance of broiler chickens. Moreover, the requirement of digestible lysine, methionine and threonine for broiler chicken during the starter and finisher periods were obtained in this study.