In the last decade, the discovery and cloning of the cyclin-dependent kinases (cdks), key regulators of cell cycle progression, have led to the identification of novel modulators of cdk activity. Initial experimental results demonstrated that these cdk modulators are able to block cell cycle progression, induce apoptotic cell death, promote differentiation, inhibit angiogenesis, and modulate transcription. Alteration of cdk activity may occur indirectly by affecting upstream pathways that regulate cdk activity or directly by targeting the cdk holoenzyme. Two direct cdk modulators, flavopiridol and UCN-01, are showing promising results in early clinical trials, in which the drugs reach plasma concentrations that can alter cdk activity in vitro. Although modulation of cdk activity is a well-grounded concept and new cdk modulators are being assessed for clinical testing, important scientific questions remain to be addressed. These questions include whether one or more cdks should be inhibited, how cdk inhibitors should be combined with other chemotherapy agents, and which cdk substrates should be used to assess the biologic effects of these drugs in patients. Thus, modulation of cdk activity is an attractive target for cancer chemotherapy, and several agents that modulate cdk activity are in or are approaching entry into clinical trials.