Based on its Greek roots, the term pharmacodynamics literally means drug power. There is, however, still some confusion in the use of the term because there is no clearly accepted definition. If pharmacokinetics is the movement or kinetics of drugs, then pharmacodynamics, in its most common usage, is the action or “power” of drugs. More specifically, pharmacodynamics has been defined as the study of the biological effects of the interaction between drugs and biological systems. 1 One can think of pharmacokinetics as what the body does to the drug and pharmacodynamics as what the drug does to the body. Figure 1 illustrates how pharmacokinetics and pharmacodynamics are determinants of the observed pharmacologic response of a drug. This same figure also demonstrates the limitation of standard pharmacokinetic investigations which stop short of assessing the actual consequences or effects (efficacy/toxicity) of the observed drug concentrations. This limitation was often ignored for several years after the widespread application of pharmacokinetic principles in clinical investigations. However, since the late 1970s, there has been an increased emphasis on combining pharmacokinetics and pharmacodynamics in the clinical evaluation of drugs. This has resulted in a large increase in the body of knowledge on the relationship between pharmacokinetics and pharmacodynamics, particularly for cardiovascular drugs. The recent proposal for the development of randomized concentration-controlled trials, as opposed to the more traditional randomized dose-controlled trials, is another example of the increased emphasis given to the pharmacokinetic-pharmacodynamic relationship in the evaluation of drugs. 2