Alkaline phosphatases (APs) are members of a multigene family, that in humans include four different genes. Their wide distribution in nature, ranging from bacteria to man, indicates that APs are involved in fundamental biochemical processes. Information on the primary structure of eukaryotic APs is accumulating very rapidly. There is a high degree of similarity between the eukaryotic APs and Escherichia coli AP. Structural comparisons with the E. coli enzyme have helped identify those residues that may participate in the active site pocket, as well as predict functional-structural features unique to eukaryotic APs. The general structure of the AP genes has now been revealed through the cloning of the germ cell AP gene in humans. The entire nucleotide sequence of the gene reveals the existence of 11 exons interrupted by 10 small introns. Elucidation of the mechanism of regulation and tissue-specific expression of AP genes will be highly relevant to understanding the re-expression of these enzymes in testicular and ovarian tumors. Two vitally important developmental processes, i.e., germ cell differentiation and early embryogenesis, provide experimentally accessible models to attempt to unravel the elusive function of APs.