M. pneumoniae infection occurs world-wide and is the most common cause of community-acquired pneumonia (CAP) in the 5 to 20 year-old age group. The most reliable diagnostic test is enzyme immunoassay that allows immunoglobulin (Ig)G and IgM titration and presents 92% sensitivity and 95% specificity on paired samples. Potentially active drugs are tetracyclines, macrolides, ketolides, lincosamides, streptogamines, chloramphenicol, and fluoroquinolones. The incidence of Legionella infection, in spite of its world-wide diffusion, is highly variable in different studies, ranging from 1% to 27% of CAP. The most likely mode of transmission is direct inhalation from Legionella-contaminated water-supply systems. Extrapulmonary manifestations are relatively common but nonspecific. However, some signs and symptoms may raise the suspicion of Legionella infection: a sputum Gram stain with a high number of neutrophils without any organism, hyponatremia, and diarrhea in a critically ill patient. Urinary radioimmunoassay (RIA) antigen detection is the method of choice for L. pneumophila serogroup 1. The best treatment regimen is a full three-week treatment with a macrolide (erythromycin, clarithromycin, azithromycin). An alternative treatment regimen may be the association of second generation fluoroquinolones with tetracyclines. A notable improvement in most of the new fluoroquinolones is their activity against Legionella, so that their use as single agent may be hypothesised even if clinical data are still insufficient for a definitive indication. Chlamydia pneumoniae account for 6-20% of CAP depending on several factors such as setting of the studied population, age group examined, and diagnostic methods used. The current gold standard for serological diagnosis of acute infection is microimmunofluorescence testing. Tetracyclines and erythromycin show good in vitro activity and so far have been the most commonly employed drugs in the treatment of C. pneumoniae infection. New macrolides, ketolides, and new fluoroquinolones are other potentially effective drugs.