Cognitive deficits are common in MS but therapeutic options are lacking. Modafinil has been show to impact cognitive performance in some trials for MS-related fatigue, but it has a variable impact in different patients. The goal of this pilot phase II double-blind placebo controlled trial is to assess the efficacy and safety of modafinil in improving self-reported cognitive function. We will also try to use phMRI to identify predictors of response to modafinil.