Protein tyrosine phosphatases (PTPs) are essential regulators of signal transduction pathways, and control, together with protein tyrosine kinases (PTKs), the reversible phosphorylation of tyrosine residues. PTPs are, from a structural biology viewpoint, one of the most comprehensively covered protein families. This chapter focuses on the structural comparison of catalytic domains of the major PTP families, and summarizes implications of the available structural data for the understanding of PTP regulation and for the design of specific PTP inhibitors. Modular domains outside the catalytic domain also significantly regulate PTP function. Inactivation of PTPs by receptor dimerization is discussed as a key regulatory mechanism of PTP activity. This control mechanism was first suggested based on structural studies of the membrane proximal D1 domain of RPTP α. More structural data regarding phosphatases in complex with their substrates and regulators will be available in the near future, which will allow a more detailed insight into the regulation and substrate recognition of this diverse class of enzymes.