The cell cycle, consisting of four distinct phases (G1, S, G2, and M) is controlled by numerous mechanisms ensuring correct cell division. Cyclin-dependent kinases (CDKs; EC 2.7.11.22) are a family of protein kinases first discovered for their role in regulating the cell cycle. Their kinase activity requires the binding of a regulatory cyclin subunit, upon which CDKs phosphorylate their substrates on serine and threonine residues. Since cyclins are synthesized and destroyed at specific times during the cell cycle, CDK kinase activity is regulated in a timely manner. Tumor associated mutations frequently deregulate certain CDK–cyclin complexes, resulting in either continued proliferation or unscheduled re-entry into the cell cycle, two properties characteristic of most human tumor cells. If it is possible to selectively interrupt the cell cycle regulation in cancer cells by interfering with CDK action, the cell will die. Therefore, the development of CDK inhibitors (CKIs) is of great interest.