Juvenile Idiopathic Arthritis (JIA) Registry

Inclusion Criteria: * For a patient enrolling into the HUMIRA® arm; a pediatric patient diagnosed at any time with moderately to severely active polyarticular or polyarticular-course JIA (defined as arthritis affecting \>= 5 joints at the time of diagnosis of polyarticular or polyarticular-course JIA) who has been prescribed HUMIRA® therapy according to the local approved HUMIRA® product labeling and meets one of the following criteria: * Enrolled patients are 4 to 17 years of age as per approved HUMIRA® product label with the addition of JIA patients 2 to \< 4 years of age in countries with available local approval for this group of patients at the time of consent to the registry. * Newly initiated (within 24 months of registry entry) on HUMIRA® therapy and has received continuous (no more than 70 consecutive days off drug) HUMIRA® therapy, and the physician can provide available source documentation of SAEs, AEs of Special Interest, and dosing information since initiation of therapy; * Or is entering after participation (within 24 months of registry entry or, if longer, continuously treated at the same site) in an AbbVie Humira sponsored study, regardless of age or the number of joints with symptoms of JIA, and has received continuous (no more than 70 consecutive days off drug) HUMIRA® therapy and the physician can provide available source documentation of SAEs, AEs of Special Interest, and dosing information since initiation of therapy. * For a patient enrolling into the MTX arm; a pediatric patient diagnosed at any time with moderately to severely active polyarticular or polyarticular-course JIA (defined as arthritis affecting \>= 5 joints at the time of diagnosis of polyarticular or polyarticular-course JIA) who is prescribed MTX therapy alone or in combination with other disease modifying anti-rheumatic drugs (DMARDs) according to the local product labeling (initiated treatment within 24 months of registry entry) and has received continuous therapy and the physician can available provide source documentation of SAEs, AEs of Special Interest, and dosing information since initiation of therapy. * Patients who were treated in the MTX arm of this registry and prematurely discontinued from the MTX arm due to being a non-responder, or became intolerant of MTX treatment or are in need of combination treatment with HUMIRA® therapy may be eligible to enroll into the HUMIRA® treatment arm if all ongoing AEs/SAEs have been resolved, and they meet inclusion criteria and can enroll within the registry enrollment period. In case of ongoing AEs/SAEs at the time of the treatment arm switch, the AbbVie Designated Physician should be contacted to assess the eligibility of patient to roll into Humira treatment arm. * Parent or guardian has voluntarily signed and dated an informed consent/patient authorization form, approved by an Institutional Review Board (IRB)/Independent Ethics Committee (IEC) if applicable according to local law, after the nature of the registry has been explained and the patient's parent or legal guardian has had the opportunity to ask questions. Pediatric patients will be included in all discussions as per applicable local regulations in order to obtain verbal or written assent. Exclusion Criteria: * Patients should not be enrolled into the HUMIRA® or Methotrexate (MTX) arm if they cannot be prescribed and treated in accordance with the approved local HUMIRA® and/or with the local MTX product label * Patients should not be enrolled into the HUMIRA® or MTX arm if they require on-going treatment with Kineret® (anakinra), Orencia® (abatacept), Rituxan® (rituximab), Enbrel® (etanercept), and Remicade® (infliximab), or any other approved biologic agents or investigational agents. * Patients should not be enrolled into the MTX arm if they have had prior treatment with any investigational agent or anti-rheumatic biologic therapy such as, but not limited to, Orencia® (abatacept), Enbrel® (etanercept), Remicade® (infliximab), Rituxan® (rituximab), or Actemra® (tocilizumab)