Experimental studies have documented that both the toxicity and the antitumor activity of many cancer drugs are time dependent. Early clinical trials have confirmed this observation for several drugs. The basic concepts of chronobiology and its application to pharmacology are reviewed. As an example, clinical trials of circadian fluoropyrimidine delivery are reviewed. Other clinical results are presented in table form. The mechanisms pertinent to the circadian time dependence of fluoropyrimidines are discussed.