This review summarizes the information that supports the potential importance of anaerobic bacteria in tonsillitis. Some anaerobic bacteria possess interfering capability with Group A beta-hemolytic streptococci (GABHS) and other pathogens. The possible role of anaerobes in the acute inflammatory process in the tonsils is supported by several observations: anaerobes have been isolated from the cores of tonsils of patients with recurrent GABHS and non-GABHS tonsillitis (NST); the recovery of anaerobes as predominant pathogens in abscesses of tonsils, in many cases without any aerobic bacteria; their recovery as pathogens in well-established anaerobic infections of the tonsils (Vincent's angina); the increased recovery rate of encapsulated pigmented Prevotella and Porphyromonas spp. in acutely inflamed tonsils; their isolation from the cores of recurrently inflamed NST; and the response to antibiotics in patients with NST. Furthermore, immune response against Prevotella intermedia is present in patients with recurrent NST, and an immune response can also be detected against P. intermedia and Fusobacterium nucleatum in patients who recovered from peritonsillar cellulitis or abscesses, infectious mononucleosis and acute non-streptococcal and GABHS tonsillitis. Although more studies are needed, these findings support the possible pathogenicity of Gram-negative anaerobic bacilli in tonsillitis.