The fundamental principles that govern drug therapy are often overlooked by the busy clinician. This disregard frequently results in the use of particular drugs and regimens that may be less ideal for the clinical situation being managed. By convention, these principles are categorized as pharmacokinetic and pharmacodynamic. Pharmacokinetic processes include drug absorption, distribution, biotransformation (metabolism), and elimination—essentially reflecting the influence of the body on the drug administered. These principles were addressed in the preceding issue of this journal. Pharmacodynamics deals with the actual mechanisms of action and effects a drug produces on the patient and is the topic for this continuing education article.