Medications administered in clinical practice obtain their therapeutic effect only to the extent that the drug is present in the appropriate concentration at the desired site. To achieve this goal, the prescribing clinician must be aware of how a drug may interact with the physiology of the patient. Pharmacokinetics is the study of this process with a focus on the absorption, distribution, metabolism, and excretion of the drug. This article reviews these four processes with a discussion of the effects of route of administration. Emphasis is given to the differences between pediatric and adult populations.