Protein kinases catalyse phospho transfer reactions from ATP to serine, threonine or tyrosine residues in target substrates and provide key mechanisms for control of cellular signalling processes. The crystal structures of 12 protein kinases are now known. These include structures of kinases in the active state in ternary complexes with ATP (or analogues) and inhibitor or peptide substrates (e.g. cyclic AMP dependent protein kinase, phosphorylase kinase and insulin receptor tyrosine kinase); kinases in both active and inactive states (e.g. CDK2/cyclin A, insulin receptor tyrosine kinase and MAPK); kinases in the active state (e.g. casein kinase 1, Lck); and kinases in inactive states (e.g. twitchin kinase, calcium calmodulin kinase 1, FGF receptor kinase, c-Src and Hck). This paper summarises the detailed information obtained with active phosphorylase kinase ternary complex and reviews the results with reference to other kinase structures for insights into mechanisms for substrate recognition and control.