This is a global registry, to evaluate the long-term safety of Humira® in patients with moderate to severe polyarticular Juvenile Idiopathic Arthritis (JIA), that are treated as recommended in the Humira® product label. Patients treated with MTX will be considered a reference group. Patients will be followed in both the Humira® and Methotrexate (MTX) arms for 10 years from the enrollment date into one of the treatment arms.