Proteins may be modified post-translationally by phosphorylation of their side-chains. The involvement of phosphoserine, phosphothreonine and phosphotyrosine is illustrated in the signaling process that leads to activation of MAP kinase and phospholysine and phosphoarginine are mentioned but the main focus of this review is phosphorylation of histidine. Phosphohistidine occurs in proteins from all eukaryotic cells studied and the amount has been estimated as 5% of total protein phosphorylation. Although phosphohistidine is well-known in prokaryotes, its role in eukaryotes has often been ignored because phosphohistidine is lost during the usual phosphoamino acid analysis procedures. In rat liver plasma membranes, a protein is phosphorylated on histidine in response to activated Ras. Several histidine kinase activities have been described in eukaryotes and one has been purified. The major protein serine/threonine phosphatases in eukaryotic cells are also very active protein histidine phosphatases. Evidence is presented for a role for histidine phosphorylation in cell signaling, possibly as a suppressor of cell proliferation.