Several investigators have demonstrated that intradental A-type (A beta and A delta) nerve fibers are responsible for the sensitivity of dentin and that the endings of the responding fibers are located in the pulp-dentin area of the tooth. The exact mode of transmission of stimuli (e.g., thermal, chemical, mechanical, etc.) across dentin, however, is still unclear, although several hypotheses have been proposed. These include direct nerve stimulation, dentinal receptor (transducer/modulation), hydrodynamic, and direct ionic diffusion hypotheses. Currently, the most accepted mechanism of intradental nerve activation associated with dentin sensitivity appears to be hydrodynamic in nature, although alternative mechanisms of transmission (e.g., direct ionic diffusion) cannot be ruled out. Recent investigations (in the cat), however, appear to provide evidence substantiating the hydrodynamic hypothesis.