In this review, we have summarized the general structural features of the catalytic subunit of cAMP-dependent protein kinase, emphasizing those features that will very likely be conserved in all members of the protein kinase family. The overall secondary structure of the catalytic core will probably be conserved throughout the catalytic core, as will the active site regions associated with MgATP binding and catalysis. The mechanisms for activation and the role of protein phosphorylation are unique for each kinase. The structure of the catalytic subunit now provides a general framework for modeling other protein kinases. Although this is no substitute for a crystal structure for each protein kinase, this one structure, nevertheless, does provide major insights to the molecular organization of each of these enzymes.