This chapter details the principles of pharmacokinetics (absorption, distribution, metabolism, and excretion) and pharmacodynamics (target effects at the receptor/enzyme) of drugs in the body. Age changes that influence pharmacokinetics and pharmacodynamics are described with a focus on important alterations in liver and kidney function, body composition, target receptors, and homeostatic reflexes. Awareness of these age-related changes is critical to drug prescribing to avoid adverse drug reactions and/or loss of efficacy.