Liquid and semi-solid formulations adherent to the mouth, pharynx and oesophagus may provide mechanical protection, palliative relief and treat local disease. These mucosae also offer a substantial area of well-perfused tissue, perhaps suitable for systemic drug delivery. However, successful formulation design has been curtailed by the lack of a generally accepted method for evaluating mucoadhesive performance. Moreover, the mucoadhesive interaction is a complex phenomenon rather than a single-process bond. Therefore, an understanding of the relationship between mucoadhesive performance and the physicochemical properties must be elucidated to allow rational design of retentive dosage forms.