This review considers those structural features of the pulp and dentine relevant to an understanding of dentine sensitivity. It does not discuss innervation, or microvasculature, which are covered in other contributions. The sensitivity of dentine is directly related to the size and patency of the dentinal tubules. Tubular occlusion by peritubular dentine deposition or the formation of other intratubular material would reduce the flow of fluid and diffusion of molecules through dentine. Irregular (reparative) secondary dentine would, because its tubules are not continuous with those of primary dentine, be expected to reduce permeability and fluid flow and decrease sensitivity. Regular secondary dentine would have little or no effect other than by increasing diffusion distances. The odontoblast layer is of limited permeability and could restrict the access of materials diffusing through the dentinal tubules to pulpal axons. Odontoblasts are not involved in the sensory process as special sensory receptors but may, by modifying the local ionic environment, alter the threshold of intradentinal nerves.