Dentin sensitivity is characterized by short sharp pain arising from exposed dentin in response to stimuli typically thermal, evaporative, tactile, osmotic or chemical and which cannot be ascribed to any other form of dental defect or disease. By this definition we understand that dentin sensitivity causes pain to patient which is sharp in intensity and short in duration. Then it says that it occurs only when dentin is exposed to the outside environment. In a healthy tooth, dentin in crown is covered by enamel and in root by cementum. Exposure of dentin may occur due to cavitation by carries or by any periodontal disease in which removal of periodontium causes exposure of root dentin. Then this exposed dentin responds to thermal, evaporative, tactile, osmotic or chemical stimuli. Enamel being a non-vital tissue, just like fingernails and hair, does not show any sensation or pain. Similarly, cementum lacks any neural tissue so does not cause any sensation or pain. However dental pulp is a vital soft tissue full of nerves and can elicit pain called pulpal pain. So dentin is the only dental tissue associated with tooth sensitivity. Now dentin is a complicated tissue. It is completely mineralized. Odontoblasts are present in the pulp but their cytoplasmic extensions, called odontoblastic processes, run through the entire thickness of dentin in a hollow canal called dentinal tubule. The peri-odontoblastic space is filled with dentinal fluid which is a continuation of the pulpal tissue fluid. Pulpal nerves from the Plexus of Raschkow, extend between the Odontoblasts and may enter the dentinal tubules for a short distance. The exact reason for dentin sensitivity is not known, but there are many theories. Direct neural stimulation theory, Transduction theory, Hydrodynamic theory. We'll start with Direct neural stimulation theory. This theory stated that pulpal nerves from the plexus of Raschkow enter the dentinal tubules and cover the full length of dentinal tubule, till the dentino-enamel junction. So when dentin gets exposed, these nerves get mechanically stimulated and hence cause pain sensation. The biggest drawback of this theory is the fact that pulpal nerves do not extend the whole length of the dentinal tubules. They terminate at the level of Odontoblasts or are limited to the inner part of dentin. Transduction theory. Now that it is known, that nerves are limited only in the inner part of dentinal tubules, this theory said that if dentin gets exposed, it is the odontoblastic process itself, that gets excited just like a nerve. This impulse then travels the odontoblastic process just like a nerve and gets transmitted to the nerve endings closely associated with the odontoblastic processes just like a nerve synapse. The excited nerve then carries this pain sensation forward. This theory was given on the basis of the fact that Odontoblasts developed from neural crest cells, so it was thought that they may have some neural properties. The drawback of this theory. Odontoblasts are not like nerve tissue, they are not capable of being excited and in producing a neural impulse. Moreover, there are no neurotransmitter vescicles in the odontoblastic process to form a synaptic junction with nerves. Hydrodynamic theory, also known as the Fluid theory, is the most accepted theory. As we know, dentinal tubules contain odontoblastic process and the space between the odontoblastic process and wall of dentinal tubule is filled with dentinal fluid. So if dentin gets exposed, this dentinal fluid directly connects the outside environment to the pulp. This theory states that, it is the dynamic behavior of this dentinal fluid in the dentinal tubules that causes sensitivity. That is, when dentin gets exposed and the dentinal tubules open to the outer environment, any stimulus, be it physical, thermal or chemical, causes movement of this fluid. For example, cold stimulus causes contraction of dentinal fluid, heat causes expansion of this fluid, air blast or consumption of sweet food act as a dehydrating substance and again causes movement of the dentinal fluid. This fluid movement then in turn mechanically stimulates the nerves present in the dentinal tubules in the deeper layer and cause dentinal sensitivity.