The dephosphorylation of proteins on their serine, threonine and tyrosine residues is catalysed by three families of protein phosphatases that regulate numerous intracellular processes. Diversity of structure within a family is generated by targeting and regulatory subunits and domains. Structural studies of these enzymes have revealed that although the two families of protein Ser/Thr phosphatases are unrelated in sequence, the architecture of their catalytic domains is remarkably similar and distinct from the protein tyrosine phosphatases. Insights into the molecular mechanisms of catalysis and regulation of these enzymes have been obtained.