Many non-receptor protein-tyrosine kinases (PTKs) function as subunits of receptors, either receptors with or without intrinsic PTK catalytic activity of their own. There are currently at least 33 known vertebrate genes that encode non-receptor PTKs. These can be divided into nine families: Abl, Fes/Fer, Syk/Zap70, Jak, Tec, Fak, Ack, Src, and Csk. Four additional non-receptor PTKs (Rlk/Txk, Srm, Rak/Frk, and Brk/Sik) do not appear to belong to any of the defined families. Here we review current knowledge of the general roles of non-receptor PTKs, as well as the characteristic features and functions of each family and its family members.