The effects of dietary phytate and microbial phytase supplementation on amino acid availability in poultry diets are of considerable practical importance. Published data on the influence of exogenous phytase supplementation on the ileal digestibility of amino acids in poultry are reviewed in this paper, with emphasis on the factors causing variability in amino acid responses. Several modes of action have been proposed by researchers to explain the influence of phytate on protein digestion, but it is hypothesised that the de novo formation of binary protein-phytate complexes in the gut under acidic conditions in the proventriculus may be the main mechanism whereby phytate depresses the digestibility of dietary amino acids as bound protein is refractory to pepsin digestion. It is also likely that phytate promotes the flow of endogenous amino acids. The review demonstrates that the choice of inert marker used in the digestibility assays is a major factor responsible for the variable phytase responses reported in the literature. The important diet-related causes for the variability include differences among ingredients, dietary levels of Ca and nonphytate P and dietary electrolyte balance, and clearly amino acid responses with added phytase may be enhanced by considering these issues in poultry feed formulations. Phytase feed enzymes, however, do not degrade the majority of dietary phytate in poultry and consequently the negative influence of phytate on protein digestibility is not completely removed by phytase supplementation. It is proposed that the experimental use of ‘dephytinised’ feed ingredients may be useful to define the actual extent to which amino acid digestibility is compromised by dietary phytate. The efficacy of current phytase feed enzymes may be further enhanced by the simultaneous use of other exogenous enzymes, which complement their activity, increase substrate access and/or absorption of liberated nutrients.