The reader is referred to numerous reviews on specific protein kinases (PKs) and groups of protein kinases which have appeared over the last few years. 1 17 Although this review will focus on new knowledge in order to give an accurate account of this subject, some repetition is unavoidable. Furthermore, it might appear to those unfamiliar with the subject that the protein kinase field is extremely broad and diverse, which is one reason to limit this review to include details of only a few protein kinases which have been more fully characterized. Another reason is that it has been realized that most, if not all, of the kinases which have been described are derived from a common ancestor in the course of evolution. The first suggestion of homology between any two protein kinases was that made in 1977 for the adenosine 3′:5′-cyclic phosphate (cAMP)- and cyclic guanosine monophosphate (cGMP)-dependent protein kinase (cAK and cGK). 18 , 19 Although this thought doubtless occurred to many scientists, the broadness of this family was not apparent at the time. It seems appropriate to use the theme of homology throughout this review as a mechanism to unify an otherwise complex subject. Thus, it might be expected that a certain physical or kinetic feature found for one protein kinase would also be present in other PKs, albeit in perhaps an altered form.