Over the last decade and a half, several disease-modifying therapies (DMTs) have been approved for the treatment of multiple sclerosis (MS) including glatiramer actetate (GA; Copaxone), interferon beta (IFNB)-1a (Avonex, Rebif), IFNB-1b (Betaferon/Betaseron), mitoxantrone (Novantrone), and natalizumab (Tysabri). Randomized controlled trials (RCTs) of each of these DMTs have demonstrated that treatment has a favourable impact on at least one (often several) of the short-term outcome measures typically used to assess efficacy in MS clinical trials. These outcomes include clinical measures of disease activity such as the number or frequency of relapses, the time to first relapse, etc. They also include clinical measures of disease severity such as disease progression on the Expanded Disability Status Scale (EDSS) or the MS Functional Composite score (MSFC), determined either as a confirmed change over a 3 to 6 month interval or as a total change over the entire duration of the trial, in addition to various magnetic resonance imaging (MRI) measures such as the number and volume of T2 lesions, the number and volume of new or gadolinium (Gd)-enhancing lesions, or the number and volume of T1 dark lesions.