Periodontium is the functional unit of tissues supporting the tooth including gingiva, the periodontal ligament (PDL), the cementum and the alveolar process. The tooth and periodontium are together called as the dentoperiodontal unit. The main support of the tooth is provided by the PDL, which connects the cementum of the root to the alveolar bone or tooth socket into which the root fits. The main function of the gingiva is to protect the surrounding tissues from the oral environment. This chapter covers the gingiva, tooth-supporting structures, periodontal ligament, alveolar bone, ankylosis, and cementum along with their definition, functions, classifications, composition and structure.