Protein tyrosine phosphorylation is a prevalent post-translational modification that can modulate enzyme activity, protein localization, protein stability, and protein-protein interactions. Protein tyrosine phosphatases (PTPs) make up a large family of over 100 enzymes in humans that catalyze the dephosphorylation of tyrosine residues on proteins. These enzymes counteract protein tyrosine kinases, which phosphorylate proteins on tyrosine residues. Many diseases, including cancers, arise from the dysregulation of tyrosine phosphorylation, and tyrosine phosphatases have been proposed as candidate drug targets for the treatment of these diseases. For example, the phosphatase PTP1B has been linked to diabetes and obesity, as it acts as a negative regulator of the insulin receptor signaling pathway by dephosphorylating the insulin receptor and its substrates. The phosphatase SHP2 has been shown to promote oncogenic signaling downstream of receptor tyrosine kinases. Both of these enzymes have been the targets of extensive preclinical drug discovery campaigns, and allosteric inhibitors of SHP2 are currently in clinical trials. Despite their physiological importance, the regulatory mechanisms and biological roles of many PTPs remain poorly defined, in part due to the lack of robust chemical tools to selectively inhibit PTPs or monitor their activity in cells. Most tyrosine phosphatases have a catalytic cysteine that is directly responsible for substrate dephosphorylation. They also have a catalytic arginine on the same loop as the cysteine residue (P loop), as well as conserved aspartic acid and glutamine residues on the WPD and Q loops, respectively. These residues make up the conserved catalytic core of classical PTPs (Figure ). The WPD loop can adopt an open or closed conformation that is dependent on substrate or inhibitor binding (Figure , left panel), and it has been suggested that this loop plays a regulatory role. In some phosphatases, such as SHP2, auxiliary non-catalytic domains physically occlude the active site to regulate activity. Furthermore, the catalytic cysteines of most PTPs are susceptible to reversible oxidation by reactive-oxygen species, and oxidation has been proposed as a dynamic regulatory mechanism. Other uncharacterized regulatory mechanisms likely exist for PTPs, making these enzymes an opportune target class for activity-based profiling. In this context, it is noteworthy that the catalytic cysteine in PTPs has a characteristically low pKa due to the active site environment, which should make it a good target for covalent probes.