Cellular CDK levels remain in constant excess throughout the normal cell cycle.
CDK activation requires the following:
  A. Binding of a cyclin.
  B. Phosphorylation of a conserved threonine (160 in CDK2/161 in CDK1) by CDK Activating Kinase (CAK = CDK7).
  C.  Dephosphorylation of a conserved threonine-tyrosine pair (14/15).
  D.  Unbinding of CDK inhibitory proteins (CKIs).
The reactions that induce or inhibit these processes are illustrated in figure 1.

Potential sites for regulation of cdk-cyclin complexes are shown in figure 2 (copied from the slide show in: http//www.sb-roscoff.fr/CyCell/cycleintro.html).

Cyclins and CDK inhibitors that associate with the various CDKs are shown in figure 3. (copied from the slide show in: http//www.sb-roscoff.fr/CyCell/cycleintro.html).

More detailed information can be found for CDK1, CDK2, CDK4 and CDK7.