Juvenile idiopathic arthritis (JIA) is the most common, chronic rheumatic disease of childhood, affecting approximately 16 to 150 out of 100,000 children. Juvenile, in this context, refers to disease onset before 16 years of age, while idiopathic refers to a condition with no defined cause, and arthritis is inflammation within the joint.

JIA is an autoimmune, noninfective, inflammatory joint disease, the cause of which remains poorly understood. It is characterised by chronic joint inflammation. JIA is a subset of childhood arthritis, but unlike other, more transient forms of childhood arthritis, JIA persists for at least six weeks, and in some children is a lifelong condition. It differs significantly from forms of arthritis commonly seen in adults (osteoarthritis, rheumatoid arthritis), in terms of cause, disease associations, and prognosis.

The prognosis for children with JIA has improved dramatically over recent decades, particularly with the introduction of biological therapies and a shift towards more aggressive treatment strategies. JIA treatment aims for normal physical and psychosocial functioning, which is an achievable goal for some children with this condition.