The joints are speciaLized organs that allow for stabiLity and locomotion. The joint structures are organized to promote efficient force transfer, optimal shock absorption, and frictionless load bearing. Synovial joint is the most common type of joint in the human body and also the most commonly affected by arthritis. Synovial joint is characterized by the presence of synovial tissue (synovium) which produces synovial fluid. The synovium covers the joint capsule and the tendon sheaths but does not cover the hyaLine articular cartilage or menisci. This is important, both in health and in disease, because the articular cartilage and menisci are avascular and are dependent on the synovial fluid for their nutritional needs. Pathological processes that affect the joint components or interfere with optimal joint nutrition can result in abnormal joint biomechanics, produce pain, and ultimately disabiLity. History and physical examination are essential for the evaluation of a patient with musculoskeletal complaints. Synovial fluid analysis and specific imaging modaLities can help focus and better characterize the pathologic process responsible for the patients' symptoms and physical findings. Occasionally, synovial biopsy is necessary to make certain diagnoses.