Main article: Dentin
Dentin is the substance between enamel or cementum and the pulp chamber. It is secreted by the odontoblasts of the dental pulp.[13] The formation of dentin is known as dentinogenesis. The porous, yellow-hued material is made up of 70% inorganic materials, 20% organic materials, and 10% water by weight.[14] Because it is softer than enamel, it decays more rapidly and is subject to severe cavities if not properly treated, but dentin still acts as a protective layer and supports the crown of the tooth.

Dentin is a mineralized connective tissue with an organic matrix of collagenous proteins. Dentin has microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp cavity to the exterior cementum or enamel border.[15] The diameter of these tubules range from 2.5 µm near the pulp, to 1.2 µm in the midportion, and 900 nm near the dentino-enamel junction.[16] Although they may have tiny side-branches, the tubules do not intersect with each other. Their length is dictated by the radius of the tooth. The three dimensional configuration of the dentinal tubules is genetically determined.Amelogenesis imperfecta is a condition in which enamel does not form properly or at all.[72] Dentinogenesis imperfecta is a condition in which dentin does not form properly and is sometimes associated with osteogenesis imperfecta.[73] Dentin dysplasia is a disorder in which the roots and pulp of teeth may be affected. Regional odontodysplasia is a disorder affecting enamel, dentin, and pulp and causes the teeth to appear "ghostly" on radiographs.

The fusion of two deciduous teeth.Gemination occurs when a developing tooth incompletely splits into the formation of two teeth. Fusion is the union of two adjacent teeth during development. Concrescence is the fusion of two separate teeth only in their cementum. Accessory cusps are additional cusps on a tooth and may manifest as a Talon cusp, Cusp of Carabelli, or Dens evaginatus. Dens invaginatus, also called Dens in dente, is a deep invagination in a tooth causing the appearance of a tooth within a tooth. Ectopic enamel is enamel found in an unusual location, such as the root of a tooth. Taurodontism is a condition where the body of the tooth and pulp chamber is enlarged, and is associated with Klinefelter syndrome, Tricho-dento-osseous syndrome, Triple X syndrome, and XYY syndrome.[71] Hypercementosis is excessive formation of cementum, which may result from trauma, inflammation, acromegaly, rheumatic fever, and Paget's disease of bone.[71] A dilaceration is a bend in the root which may have been caused by trauma to the tooth during formation. Supernumerary roots is the presence of a greater number of roots on a tooth than expected.