Dentin is that mineralized tissue that forms the bulk of the tooth Anatomically speaking it forms Junction with enamel, the Dentino-enamel Junction and with Cementum, the Cemento-dentinal Junction. On the inside it forms the walls of the Pulp chamber and Pulp canals. If we see its formation, during the stages of tooth development in the Advanced Bell Stage, the cells of the Dental Papilla differentiate into Odontoblasts, these Odontoblasts deposit dentin. On observing the tooth microscopically, the Odontoblasts present adjacent to the Dentin in the pulp chamber. Dentinal tubules run from the pulp chamber to the outer surface carrying the Odontoblastic processes within them. Coming to its classification, the Dentin has been classified into three types Primary Dentin, Secondary Dentin and Tertiary Dentin. Let us start with Primary Dentin. By definition primary Dentin is the Dentin which is formed before root completion. When we study chronology of teeth, we know that Dentin formation begins when tooth development enters Advanced Bell Stage and it ends at the stage of Root Completion, the dentin formed during this period is Primary Dentin. Now Primary Dentin is of two types, first Mantle Dentin. Mantle dentine is the first formed Dentin, it is present in the crown just beneath the Dentino-enamel Junction. It is around 20 microns wide. On observing microscopically we see that in the Mantle Dentin the collagen fibers run perpendicular to the Dentino-enamel Junction. These are made of type 3 collagen, are around 0.1 to 0.3 microns in diameter and are known as Kroff's fibers. Mantle dentin is mineralized only by Matrix Vescicles in a Globular Pattern done at a slower rate and does show lesser defects. The second type of Primary Dentin is circumpulpal Dentin. Circumpulpal Dentin is the dentin that forms the bulk of Dentin it forms both coronal dentin and radicular dentin. On observing microscopically, collagen fibers are arranged randomly and packed more closely. They are made of type 1 collagen with a smaller diameter of around 0.05 microns. In comparison to Mantle Dentin, Circumpulpal dentin is more mineralized, done in both Globular pattern and Linear pattern. Defects like Inter-globular Dentin are frequently seen in Circumpulpal dentin. Next, Secondary Dentin. By definition, Secondary Dentin is the dentin formed after root completion. That is Primary Dentin formation ends at root completion, but dentin formation continues till the death of the pulp tissue. This dentin is Secondary dentin. As long as Odontoblasts are present in the pulp they continue depositing Secondary dentin. In comparison to Primary Dentin the rate of deposition of Secondary Dentin is slow. Moreover, within the tooth the rate may differ, like it is high it roof and floor of the pulp chamber. Finally, Tertiary Dentin. Tertiary Dentin is deposited only in response to a mechanical injury or pathology. In case of an external stimulus, like caries or trauma, when the Odontoblast or their processes get damaged, a localized deposition of dentin takes place in an attempt to protect the pulp and seal of the zone of injury. This is Tertiary Dentin. In comparison to Primary and Secondary Dentin the rate of deposition of Tertiary Dentin is much faster, due to this rapid deposition Tertiary dentin is less organized. Like, there may be lesser number of dentinal tubules or dentinal tubules are wider moreover, they may have an irregular course. Sometimes Odontoblast may get entrapped in the dentin making it Osteodentin. Tertiary dentin can be of two types. These types are on the basis of the Odontoblast that deposit Tertiary Dentin. In the first type, when external injury occurs the Odontoblastic processes get injured but the Odontoblasts survive these Odontoblasts then start depositing Tertiary Dentin. This type of Tertiary dentin is called Reactionary Dentin. In the second type the external injury is severe and causes the degeneration of Odontoblastic processes and the Odontoblasts. Then from the Cell-Rich Zone in the pulp, the Undifferentiated Mesenchymal Cells start migrating to the place of degenerated Odontoblasts and differentiate into Odontoblasts. These newly differentiated Odontoblasts then deposit Tertiary Dentin which in this case is called Reparative Dentin.