Protein kinases play a central role in the regulation of the eukaryotic cell cycle. Recent research has concentrated on a particular family of protein kinases, the cyclin-dependent kinases (CDKs), and their involvement in regulating particular cell cycle transitions, such as the initiation of DNA synthesis (S phase) or of cell division (mitosis). One can think of these enzymes as the basic machinery of the cell cycle; their activity is then modulated by proteins which transduce signals from the external environment, and by proteins that monitor the progress of events such as DNA replication or the formation of the mitotic spindle. This review will be structured so as to introduce the cyclin-CDK motif, outline which cyclin-CDKs are involved at different cell cycle stages, their direct regulation by other protein kinases and phosphatases, and lastly the importance of other protein kinases in the cell cycle.