Necrotizing gingivitis (NG) or necrotising ulcerative gingivitis (NUG) is considered to be an acute opportunistic gingival infection caused by bacterial plaque. It appears more frequently in undernurished children and young adults as well as patients with immunodeficiency. In its pathogenesis, there are factors related to the oral microbiota with invasion processes on the one hand, and on the other hand, factors associated with the host, such as signs of capillary and immunological disorders as well as undernurishment. The disease is characterized by pain, bleeding and papillary necrosis with tendency to relapse. Diagnosis is made by a simple clinical examination. However, complementary tests ought to be performed in order to eliminate the possibility of illnesses systemic or immunodeficiency. Early and sustained treatment is strongly recommended. Lesions of the gums (craters in the interdental papillae) as an aftermath of the disease is a possibility, or if there is necrotizing periodontitis there will be loss of attachment tissue.