The primary goal of bioadhesive controlled drug delivery is to localize a delivery device within the body to enhance the drug absorption process in a site-specific manner. Bioadhesion is affected by the synergistic action of the biological environment, the properties of the polymeric controlled release device, and the presence of the drug itself. The delivery site and the device design are dictated by the drug's molecular structure and its pharmacological behaviour. This review addresses several issues which clarify the central goals of bioadhesive drug delivery research. Subsequently, a brief review of a wide variety of techniques which have been used to characterize bioadhesives is presented. This is followed by an overview of several proposed mechanisms of adhesion. The primary mechanisms for polymer systems, adsorption and diffusion, are examined in more detail.