Professor Saltzman reviews the pharmacokinetic first-order rate equation that can be used to model changes in drug concentration in the blood, as well as its derivation from the law of conservation of mass. The importance of maintaining a drug concentration that is sufficient for therapeutic purpose, but below a toxic level, is emphasized. Since this is directly affected by drug administration method, ways to localize and sustain therapeutic concentrations of drug, such as incorporating in slow-releasing, biocompatible polymers are introduced. Professor Saltzman gave some examples of clinical applications of controlled release drug delivery system, such as anti-restenosis drug incorporated into stents, and chemotherapeutic drugs in brain implants and microspheres.