Pediatric Arthritis Study of Certolizumab Pegol

Inclusion Criteria: * Study participant is 2 to 17 years of age (inclusive) at Baseline (Visit 2) * Study participants must weigh ≥10 kg (22lb) at Baseline (Visit 2) * Study participants must have had onset of signs and symptoms consistent with a diagnosis of Juvenile Idiopathic Arthritis (JIA) (according to the International League of Associations for Rheumatology Classification of Juvenile Idiopathic Arthritis, 2001) and initiation of JIA treatment for at least 6 months prior to Baseline (Visit 2). Eligible JIA categories include: polyarthritis rheumatoid factor-positive, polyarthritis rheumatoid factor-negative, extended oligoarthritis, juvenile psoriatic arthritis, and enthesitis-related arthritis (ERA) * Study participants must have active polyarticular-course disease, defined as ≥5 joints with active arthritis at Screening and at Baseline * Study participants must have had an inadequate response to, or intolerance to, at least 1 disease-modifying antirheumatic drug (DMARD) (nonbiologic or biologic). For example, study participant had prior inadequate response to methotrexate (MTX) (based on the Investigator's clinical judgment) * If the study participant is using MTX, then the study participant must have been on MTX for a minimum of 3 months at Screening. In addition, the dose must have been stable for at least 1 month before Screening at ≥10 to ≤15 mg/m\^2 per week. If the study participant is not using MTX, then the treatment must have been previously withdrawn for d…