The structure of the catalytic subunit of cAMP-dependent protein kinase, the first protein kinase structure to be solved, is reviewed. The general architecture of the enzyme is described as well as the active site regions associated with substrate binding and catalysis. In particular, the unique features of the protein kinase nucleotide fold are outlined. While the catalytic subunit is one of the simplest of the protein kinases, it nevertheless serves as a structural framework for the catalytic core of the entire protein kinase family which now includes over 200 important regulatory enzymes. The essential and conserved features of this core are summarized, and a preliminary model of myosin light-chain kinase, based on the structure of the catalytic subunit, is also discussed.