Subjects who are eligible for the 26 week double blind phase will be randomized (1:1 ratio) to either active tofacitinib or placebo. For subjects with polyarticular course JIA (ie, extended oligoarthritis, polyarthritis RF+, polyarthritis RF , systemic JIA with active arthritis but without active systemic features), randomization will be stratified by JIA category and baseline CRP (normal, above normal). For subjects with psoriatic and enthesitis related arthritis, randomization will be stratified by JIA category.