Anaerobic infections are common and some are serious with a high mortality rate. The clinically important anaerobes are several genera of Gram-negative rods (Bacteroides, Prevotella, Porphyromonas, Fusobacterium and Bilophila). Among the Gram-positive anaerobes, there are cocci (primarily Peptostreptococcus) and sporeforming (Clostridium) and non-sporeforming bacilli (especially Actinomyces and Propionibacterium). In terms of frequency, there are four major sites of anaerobic infection - pleuropulmonary, intra-abdominal, female genital tract, and skin and soft tissue infections (with or without involvement of underlying bone). Infectious diseases caused by these anaerobes include following symptoms, bacteremia, brain abscess, neck space infections and infections following head and neck surgery, chronic sinusitis and otitis media, dental and oral infections, pleuropulmonary infections, intra-abdominal infections, female genital tract infections, non-puerperal breast abscesses and diabetic foot ulcers.