This review summarizes the microbiology, diagnosis and management of infective endocarditis (IE) due to anaerobic bacteria. Anaerobic bacteria are an uncommon but important cause of IE. Most cases are caused by the anaerobic and microaerophilic streptococci Propionibacterium acnes, Bacteroides fragilis and Clostridium species. Predisposing factors and signs and symptoms of IE caused by anaerobic bacteria are similar to those seen in IE due to facultative anaerobic bacteria with the following exceptions: there is a lower incidence of pre-existing valvular heart disease, a higher incidence of thromboembolic events and a higher mortality rate with anaerobic IE. The gastrointestinal tract is the most common source for B. fragilis group IE, the head and neck are the most common origin for Fusobacterium and Bacteroides species, and the head and neck or genitourinary tract is the most common source for Peptostreptococcus species. Multiple mycotic aneurysms are a common complication with anaerobic IE. Others include valvular destruction, aortic-ring abscess, aortitis, cardiogenic shock, dysrrhythmias and septic shock. The mortality rate for patients with anaerobic IE ranges from 21 to 43%. Treatment of anaerobic IE includes the use of antimicrobial effective against these organisms. Surgical intervention may be indicated in some patients.