The joint capsule is vital to the function of synovial joints. It seals the joint space, provides passive stability by limiting movements, provides active stability via its proprioceptive nerve endings and may form articular surfaces for the joint. It is a dense fibrous connective tissue that is attached to the bones via specialised attachment zones and forms a sleeve around the joint. It varies in thickness according to the stresses to which it is subject, is locally thickened to form capsular ligaments, and may also incorporate tendons. The capsule is often injured, leading to laxity, constriction and/or adhesion to surrounding structures. It is also important in rheumatic disease, including rheumatoid arthritis and osteoarthritis, crystal deposition disorders, bony spur formation and ankylosing spondylitis. This article concentrates on the specialised structures of the capsule--where capsular tissues attach to bone or form part of the articulation of the joint. It focuses on 2 joints: the rat knee and the proximal interphalangeal (PIP) joint of the human finger. The attachments to bone contain fibrocartilage, derived from the cartilage of the embryonic bone rudiment and rich in type II collagen and glycosaminoglycans. The attachment changes with age, when type II collagen spreads into the capsular ligament or tendon, or pathology--type II collagen is lost from PIP capsular attachments in rheumatoid arthritis. Parts of the capsule that are compressed during movement adapt by becoming fibrocartilaginous. Such regions accumulate cartilage-like glycosaminoglycans and may contain type II collagen, especially in aged material.(ABSTRACT TRUNCATED AT 250 WORDS)