The cell cycle is regulated by the activity of cyclin dependent kinases (CDKs) associated with cyclins. Their activity is opposed by various CDK inhibitory proteins. Disregulation of the cell cycle occurs in various pathological conditions - the most important of them being cancer. Therefore, there is a continuous effort to search for chemical CDK inhibitors, which may be used as antiproliferative drugs. Various compounds have been already isolated and synthesized (e.g. olomoucin, roscovitine, flavopiridol and indirrubin). All known CDK inhibitors compete with ATP in the binding with CDKs. Chemical CDK inhibitors show antitumor activity in various in vitro assays and on experimental animals. Flavopiridol is already being tested in phase II clinical trials.