Septic arthritis has shown no change in incidence, and despite advances in antimicrobial therapy is often responsible for residual functional impairment and for a high mortality rate among debilitated patients. Risk factors include older age, diabetes mellitus, rheumatoid arthritis, immunodeficiency, and a preexisting joint disease (e.g., rheumatoid arthritis) to which the symptoms of septic arthritis are sometimes ascribed. Staphylococcus aureus contributes over two-thirds of identified organisms; a range of streptococci and gram-negative bacilli are next in frequency. The most common site is the knee, followed by the hip and shoulder. Over 10% of patients have polyarticular involvement reflecting bacteremia and diminished resistance to infection; (over 50% of polyarticular forms occur in rheumatoid arthritis patients). Prosthetic joint infection is becoming increasingly common; chronic forms due to intraoperative contamination and resulting in septic loosening should be distinguished from acute hematogenous infection in which emergency treatment can allow to salvage the prosthesis. Demonstration of the organism in the joint is the key to the diagnosis. Joint aspiration should be performed on an emergency basis, if needed after identification of radiographic landmarks or under ultrasonographic guidance. Seeding the fluid on blood culture flasks immediately after aspiration increases the yield. Antibiotics should be started as soon as the microbiological specimens have been collected. When aspiration is difficult (hip) or inadequate, arthroscopic drainage usually makes arthrotomy unnecessary. Early antiinflammatory therapy (nonsteroidal antiinflammatory drugs, systemic or local glucocorticoids, anticytokines, and antiinflammatory cytokines) are being considered as tools for limiting joint damage; their efficacy and safety will first have to be established in animal studies.