 iTOP is a retrospective and prospective Irish observational study of participants receiving natalizumab, with each participant to be followed for 3 years. This study is designed to address the long-term safety profile and the long-term impact on disease activity and progression of natalizumab with marketed use. Collection of efficacy and safety data at 6- monthly intervals to coincide with regular clinic visits and routine clinical practice will therefore be undertaken during the iTOP observational period. 