All available retrospective and prospective data from pediatric MS participants who have received at least 1 dose of natalizumab before turning 18 years old and before 31Mar2015 will be used in this study. Prospective data are defined as data to be collected in the future, i.e., from new participants not included in the first data transfer or additional data from existing participants that were not included in the first data transfer. Existing biospecimen samples will be evaluated but not collected as part of this study.