Necrotizing periodontal diseases are unique in their clinical presentation and course. Data suggest that the etiology and pathogenesis of necrotizing periodontal diseases may also be distinctive from other periodontal diseases. Necrotizing ulcerative gingivitis (NUG) is a type of necrotizing periodontal disease in which the necrosis is limited to the gingival tissues. Three specific clinical characteristics must be present to diagnose NUG, pain (usually of rapid onset) interdental necrosis, and bleeding. Epidemiological and prospective clinical studies have found an altered ability to cope with psychological stress, immunosuppression, and tobacco use to be strongly associated with the onset of NUG.