Chronic arthritis, the most common chronic rheumatic disease of childhood, is one of the most frequent chronic illness of children and an important cause of short and long term disability. It is not a single disease, but a group of related, genetically heterogeneous, phenotypically diverse immunoinflammatory disorders that affect the joints and other structures, possibly activated by contact with external antigens. Since its introduction in 1994, the term Juvenile Idiopathic Arthritis (JIA) has largely supplanted the terms Juvenile Chronic Arthritis and Juvenile Rheumatoid Arthritis. However, it is necessary to understand the older classifications in order to interpret the literature on the subject. Each subtype of JIA is to provide a general introduction.