Phytate-degrading enzymes, also known as phytases, have a wide distribution in plants, microorganisms and some animal tissues (Konietzny and Greiner, 2002; see Hill and Richardson, Chapter 5, and Mullaney and Ullah, Chapter 7, this volume). They belong to a special class of phosphomonoesterases termed myo-inositol hexakisphosphate phosphohydolases, which are capable of initiating the stepwise release of phosphate residues from phytate (salts of myo-inositol hexakisphosphate), the major storage form of phosphate in plant seeds and pollen. The ability of such enzymes to hydrolyse phytate is usually known only from in vitro assays, and information on their in vivo function is rather limited. As enzymes are classified in general by their in vivo function, the term 'phytate-degrading enzyme' is preferred here to the term 'phytase', and is used throughout the chapter. The classification of enzymes as phytases based solely on in vitro assays becomes problematic when considering enzymes such as glucose-1-phosphatase in Escherichia coli and Enterobacter cloacae. This enzyme can hydrolyse phytate, albeit slowly, in vitro even though this is clearly not its in vivo function. Unless otherwise stated, designation of an enzyme as 'phytatedegrading' is based on in vitro assay using soluble sodium phytate.