Therapeutic interferon beta is the first line treatment of relapsing remitting Multiple Sclerosis. However, despite their success in improving patient wellbeing, all IFNß products encounter a significant problem: immunogenicity. In some patients, IFNß products induce the formation of antidrug antibodies that undermine treatment efficacy and may even lead to treatment failure. These patients not only have a substantial health risk due to failing treatment, but are being treated with expensive products that no longer work. Developing low or non-immunogenic therapeutic IFNß products requires an understanding of why these products are immunogenic. Preliminary data suggest several product related factors such as formulation, aggregation and epitopes influencing the immunogenicity of IFNß-1b. Research described in this book aims to determine which of these potential triggers is causing immunogenicity. Our findings have pushed us one step closer to answering this question.