Diagnosis of septic arthritis requires aspiration and analysis of joint fluid. However, nonseptic articular disorders are fairly common and represent a significant diagnostic and therapeutic challenge. Such disorders include gout, Milwaukee shoulder, rapidly destructive articular disease, amyloid arthropathy, hemophilic arthropathy, primary synovial osteochondromatosis, pigmented villonodular synovitis, neuropathic arthropathy, and foreign-body synovitis. The clinical signs of articular disease, which include pain, swelling, and limitation of motion, are often nonspecific and can overlap with those of osseous or extraarticular disorders. Many articular processes have characteristic radiologic appearances that allow definitive diagnosis. Radiography is an important part of the evaluation of patients with articular disease. However, magnetic resonance (MR) imaging is the method of choice for characterizing the various disorders and assessing the full extent of osseous, chondral, and soft-tissue involvement. MR imaging can exquisitely demonstrate joint effusions, synovial proliferation, articular cartilage abnormalities, subchondral bone, ligaments, muscles, and juxtaarticular soft tissues. Although a wide spectrum of noninfectious processes may involve the joints, careful analysis of the imaging findings and correlation of these findings with the patient's clinical history can suggest a more specific diagnosis in most cases. Awareness and understanding of the underlying histopathologic findings aids in interpretation of MR images.