Clinical pharmacology is the study of drugs in healthy volunteers and patients and defining the relationships between dose, drug exposure, and response in populations. Drug dose refers to an amount of drug administered via a particular dose route (e.g., intravenous, oral, subcutaneous). Drug exposure is a function of the concentration of drug in the body, and usually levels in the blood/plasma/serum serve as a surrogate, with respect to time. Response is a measure of effect and can relate to both advantageous (efficacy) and untoward (toxic) reactions. The dose–response relationship for a drug is based on the relationship between dose, dose route, and drug exposure that is defined by the pharmacokinetics (ADME) of the drug (Fig. 2.1). Pharmacokinetics (PK) is defined loosely as what the body does to the drug and is comprised of the absorption, distribution, metabolism, and elimination (ADME) profile. A key principal in clinical pharmacology is studying how drug PK differs amongst populations. Initial PK studies in humans for most drugs are done in healthy, volunteer populations and the results extrapolated to and refined in patient populations.