Periodontitis is a set of inflammatory diseases affecting the periodontium (i.e. the tissues that surround and support the teeth). Periodontitis affects both the superficial structures of the periodontium (gingiva) and the deeper structures (alveolar bone and periodontal ligament).

Periodontitis is a destructive disease involving the loss of gingival attachment followed by progressive loss of periodontal ligaments and alveolar bone tissue. If left untreated, it can lead to the loosening and subsequent loss of teeth.

Similar to all periodontal diseases, periodontitis is caused by micro-organisms that adhere and grow on the teeths surfaces (bacterial plaque). It is generally agreed that periodontitis is always preceded by gingivitis.