Protein Ser/Thr phosphatases, belonging to the superfamily of protein phosphatases, are a diverse group of phosphatases that catalyze dephosphorylation and specifically remove phosphate groups from phosphorylated Ser and Thr residues within their substrates. Protein Ser/Thr phosphatases are comprised of two distinct families of phosphatases, including PPP phosphatases and PPM (or PP2C-related) phosphatases, which are coded by two unrelated gene families but share remarkably similar architecture of catalytic domains. PPP family was initially divided into two classes, type-I phosphatases (PP1) and type-2 phosphatases (PP2) consisting of PP2A, PP2B and PP2C isoforms. Novel protein phosphatases, such as PP4, PP5, PP6 and PP7, are also included in PPP family. PPM family is comprised of a number of Mg2+-dependent phosphatases encoded by different genes, which are structurally related to PP2C.