Pharmacokinetics is defined as the study of the time course of drugs and their metabolites through the body. Pharmacodynamics is defined as the study of the time course and intensity of pharmacological effects of drugs. A convenient lay description of these terms is that pharmacokinetics describes what the body's physiology does to a drug, and pharmacodynamics describes what a drug does to the body. Although clinicians are more interested in drug effects than drug concentrations, these disciplines are closely connected. Pharmacokinetic and pharmacodynamic variability is a major determinant of the dose–effect relationship in patients (Figure 8–1). There is increasing recognition that genetic variability—in the form of polymorphic genes controlling the transcription of proteins involved in drug-metabolizing enzymes, drug transporters, and drug targets—is a substantial determinant of pharmacokinetic and pharmacodynamic variability. An integrated knowledge of these areas is essential in the drug development process and can be instrumental in individualizing dosage regimens for specific patients.