The kinase Csk is the primary negative regulator of the Src‐family kinases (SFKs, e.g., Lck, Fyn, Lyn, Hck, Fgr, Blk, Yes). Csk phosphorylates a tyrosine on the SFK C‐terminal tail that mediates autoinhibition. Csk is recruited to membrane-anchored SFKs and activated allosterically via its SH2 domain, which binds SFK- phosphorylated motifs in CBP/PAG1 (Csk binding protein/phosphoprotein associated with glycosphingolipid- enriched microdomains), Paxillin, Fak, the Dok family, and other tyrosine-phosphorylated proteins. Csk also binds phosphatases, including PTPN12 (PTP‐PEST) and immune‐cell PTPN22 (LYP/Pep), which dephosphorylate SFK activation loop to promote autoinhibition. <br> Purified Csk homodimerizes in competition with PTPN22 binding through an interface that is mediated by symmetrical SH3-SH3 interactions and that overlaps the phosphatase binding footprint. Mutation in SH3 impairing homodimerization enhances PTPN22 binding.