Tyrosine phosphoprotein phosphatases (PTPases) are found ubiquitously in cytosolic and particulate fractions of cells. They are unique in terms of their substrate sensitivity, neutral pH optimum, resistance to EDTA, and inhibition by low concentrations of zinc or vandanate. PTPases exert bothpositive and negative effects on a number of metabolic and signal transduction pathways. This volume explores the sequence, structure, and biochemical properties of PTPases, regulation of PTPase expression, substrate preferences and catalytic mechanisms, physiology, covalent modification,modification by chemical agents and proteolysis, and interactions with other proteins, peptides, and macromolecules. There is an extensive bibliography to assist the reader in further research.