SUMMARY. Bacterial infection of the pleura is an old disease that continues to have a considerable mortality, of >15%. It is more common in males, and in the presence of diabetes mellitus, malignancy and alcoholism. The bacteriology of pleural infection has been changing during the past decades. Although pleural fluid culture is the «gold standard» for the identification of microorganisms in the pleural fluid, molecular methods such as polymerase chain reaction (PCR) have a notably higher sensitivity (75% versus 60%). Community-acquired pleural infection (CAPI) and hospital-acquired pleural infection (HAPI) have substantial differences in both their bacteriology and mortality, while the bacteriology of both differs markedly from that of pneumonia. The «Streptococcus milleri group» is the predominant isolate in CAPI, followed by Streptococcus pneumoniae. In HAPI the most common isolates are Staphylococcus aureus, usually methicillin-resistant (MRSA), and Enterococcus spp. Given the higher incidence of CAPI compared to HAPI, «Streptococcus milleri» accounts for the greatest number of pleural infections. The «Streptococcus milleri group» consists of Streptococcus anginosus, Streptococcus constellatus and Streptococcus intermedius. The unique characteristics of these bacteria favour the production of putrifying and necrotizing infections. The most common thoracic infection due to these organisms is empyema. Drainage of the infected pleural fluid and administration of antibiotics are essential components of the management of pleural infection. Knowledge of the pleural infection bacteriology is a useful adjunct in the selection of the appropriate antibiotic treatment. Pneumon 2009; 22(1):–