Synthetic peptides are widely used as models to study various aspects of protein structure and function. In the broad area of protein phosphorylation, synthetic peptides have been employed extensively as tools in studying various biochemical properties of protein kinases, such as determinants of substrate specificity and mechanisms of allosteric regulation. Comparable sorts of studies with protein phosphatases have been done to a limited extent, but until recently there has been a great deal of confusion regarding the identification and classification of different phosphatase activities. This confusion has made it difficult to compare different preparations of phosphatase from different laboratories and has therefore hampered efforts to exploit the various advantages of using synthetic peptides to study these enzymes.