Periodontitis refers to a number of inflammatory diseases affecting the periodontium, that is, the tissues that surround and support the teeth. Periodontitis involves progressive loss of the alveolar bone around the teeth, and if left untreated, can lead to the loosening and subsequent loss of teeth. Periodontitis is caused by microorganisms that adhere to and grow on the tooth's surfaces, along with an overly aggressive immune response against these microorganisms. This book reviews research on periodontitis including the role of the TH17 pathway in the progression of periodontal disease; an outline of the risk factors relating to the most prevalent chronic periodontal diseases and others.