This document discusses sustained release (SR) and controlled release (CR) drug delivery systems. SR systems provide medication over an extended period, usually 8-12 hours, following first-order kinetics. CR systems maintain constant drug levels by releasing the drug at a predetermined rate following zero-order kinetics. SR systems do not necessarily localize drug to the active site, while CR systems often do. SR and CR delivery can reduce side effects and dosing frequency while improving bioavailability and patient compliance compared to conventional dosage forms. Factors like dosage form materials, drug properties, and environment affect drug release from these systems.