The cyclin-dependent kinases (Cdks) have a central role in coordinating the eukaryotic cell division cycle, and also serve to integrate diverse growth-regulatory signals. Cdks are controlled through several different processes involving the binding of activating cyclin subunits, of inhibitory Cip or INK4 subunits, and phosphorylation. Crystallographic studies of Cdks in four different complexes, reviewed here, have revealed the mechanisms by which these regulatory processes control the Cdk switches. All of these mechanisms involve conformational changes in and around the catalytic cleft of the kinase, indicating that Cdks have evolved an intrinsic conformational flexibility. This flexibility is central to their ability to switch states in response to a diverse range of growth-regulatory signals.