Controlled release systems have been developed to improve the temporal and spatial presentation of drug in the body, to protect drug from physiological degradation or elimination, to improve patient compliance, and to enhance quality control in manufacturing of drug products. When designing controlled-release systems, it is important to identify and understand particular mechanisms involved in the release process. Often, more than one mechanism is involved at a given time or different mechanisms may dominate at different stages of the drug delivery process. This chapter begins with several vignettes, each highlighting a mode of controlled drug delivery and identifying associated mechanisms. An introductory description of several of the mechanisms follows. Details regarding these mechanisms are provided in subsequent chapters.