Non-spore-forming anaerobic bacteria are recognized as the most numerous and important pathogens in pyogenic liver abscess. These infections are usually polymicrobial. Frequent causes of anaerobic liver abscess are acute and chronic inflammatory bowel disease with or without perforation, malignancy and/or surgery of the gastrointestinal tract or pelvic organs, and biliary tract disease. Many abscesses are still classified as cryptogenic. A thorough understanding of bacterial etiology and the use of refined and accurate localizing and diagnostic techniques facilitate early diagnosis and effective treatment. In addition, the availability of bactericidal antimicrobial agents with extended spectra of activity against anaerobes and their use in selected cases is changing the classic therapeutic approach of antimicrobial therapy and open surgical drainage. Percutaneous aspiration has been used successfully in lieu of open drainage in several cases and antimicrobial treatment without drainage has been successful in others.