Acute septic arthritis is a medical emergency both for its diagnosis and its treatment. There are numerous predisposing factors of this condition. Staphylococcus aureus is the most frequent isolated bacteria. In 15% of cases, the clinical picture is not a monoarticular involvement, but rather a polyarticular one. Diagnosis is based on the identification of the micro-organism by joint aspiration and/or blood cultures. These bacteriological samples have to be realized before antibiotics administration. Cultures remain negative in 10 to 20% of true cases of septic arthritis. The antibiotic treatment has to be started when the direct Gram stain examination is positive (50% of cases) or if the suspicion is high but the Gram stain negative. It is frequently necessary to search for an associated endocarditis. It is necessary to remember the possibility of a gonococcal arthritis (tenosynovitis, skin lesions). Joint drainage can be achieved either by closed-needle aspiration or by surgical drainage (hip, destructive lesions on radiographs, no satisfactory response to medical aspirations). Duration of antibiotic treatment varies between 6 and 12 weeks, with the exception of gonococcal arthritis (10 days).