By definition, monoarticular arthritis means one-joint involvement, even though, in fact, such a condition is often an oligoarthritis because as many as two or three separate joints will be involved. Arthritis is often limited and may regress, so that it is frequently misdiagnosed. Sometimes, a monoarticular condition may be a polyarthritis onset (i.e., rheumatoid arthritis). Monoarticular arthritis can be caused by many factors, such as infections (septic arthritis), nonspecific inflammatory processes (reactive arthritis), crystals deposition (gout, CPPD crystal deposition disease), trauma, neoplasm (pigmented villonodular synovitis), immunologic conditions (amyloidosis) and hormonal changes (parathyroid disease). Its onset is usually acute and sometimes dramatic, with fever, pain and joint swelling, so that a decision must be made promptly to stop rapid illness evolution and to prevent the irreversible destruction of cartilage and bone (especially in septic arthritis). Diagnostic studies are performed with mono-bilateral radiographs of the joint. Radiographic findings (i.e., soft tissue swelling, joint effusion, widening and thinning of joint spaces, bone erosions and destruction of bone surface) are typical of the disease, but some findings (e.g., type of evolution and progression), laboratory tests, synovial biopsy and arthroscopy can differentiate infectious from inflammatory forms. Scintigraphy can depict isotopic joint uptake, before articular abnormalities are demonstrated with radiography, thanks to its high sensitivity; nevertheless, because of its low specificity, scintigraphy may miss some kinds of lesions (including osteoarthritis) and cannot easily differentiate osteomyelitis from septic arthritis. CT and MRI play a secondary, though not negligible, role, especially to study such deep infections as psoas abscesses, which may mimic arthritides.