Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example. and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains. the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments~ a the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic+. tudies of the structures of SH2, SH3 and tyrosine kinase domains. biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.