This section provides detailed protocols for peptide synthesis on membrane (SPOT) and describes the application of this technology to protein tyrosine phosphatase (PTPs) substrate selectivity studies. Applications include PTP binding and dephosphorylation assays on phosphotyrosine peptides derived from known substrates, such as the insulin receptor (IR) autophosphorylation site, and on peptides from focused or random SPOT peptide libraries, to discover consensus binding motifs. Weak or transient interactions that cannot be revealed by regular SPOT binding can be uncovered using SPOT double synthesis (SPOT-DS), whereby two different peptides are synthesized on the same spot. In SPOT-DS, one peptide is a substrate for the enzyme, whose conversion is indicative of a transient interaction of the enzyme with the other (variable) peptide. Using SPOT-DS, three IR regions that interact with full-length PTP-1B in a non-phosphorylation-dependent manner were revealed. In order to further study multiple interaction sites, we have developed a strategy to synthesize up to four peptides per spot: "SPOT(4)". Finally, several examples are provided that illustrate how the SPOT technology can be used in kinase and protease selectivity studies as well.