Juvenile idiopathic arthritis (JIA) is a set of inflammatory arthritic conditions defined by arthritis of unknown cause with onset in a child under age 16 years. The JIA has a worldwide distribution, with an incidence of ~. 1:1000. Although the cause is unknown, there are multiple genetic and environmental risk factors, as well as observed immunologic abnormaLities, from which we can derive clues about the pathophysiology of the disease. There are no curative treatments, but current therapy is often able to control the underlying inflammation and improve function and symptoms.