The formation of the tooth crown implies dentin and pulp construction (including mantle dentin, odontoblasts, Höhl cells, and vascular, axons and pulp cells). The root formation involves acellular and cellular cementum, peripheral and central dentins. Vascular cells and nerves are located in the central part of the root, whereas the superficial layer (Hopewell-Smith and Tomes granular layers) are located at the periphery. In the central part of the pulp, fibroblasts and immunocompetent cells are identified as T-lymphocytes, B-lymphocytes, dendritic cells, NK cells. In human deciduous teeth, HLA-DR+, CD68+, and factor XIIIa are present, whereas T and B-lymphocytes form clusters. The Hertwig’s Epithelial Enamel Epithelium (HERS’s) is formed by two layers. The HERS’s cells dissociate, and cells, taking origin from the dental follicular sac and from the papilla, migrates. The OEE becomes periodontal ligament cells whereas the IEE becomes cementoblasts. The interconversion of epithelial cells into mesenchymal cells seems to be a crucial event in tooth root formation. Tooth eruption is associated to root lengthening. The temporary tooth is gradually destroyed (root cementoclasts), whereas the permanent coronal part of the tooth erupts and acquire it final shape and position.