The investigation will involve a double-blind, placebo controlled, cross-over study examining the efficacy of armodafinil in improving neurocognitive functioning and reducing cognitive fatigue in MS. Patients who report MS-related cognitive difficulties and perform at least 1 standard deviation below the mean on a brief cognitive screen will be given a thorough neuropsychological evaluation at two time points. Half of the patients will be randomized to receive a single oral dose of lactose placebo prior to the first testing session. After a washout period of one week, they will then receive 250mg of armodafinil prior to a second testing session (P/A group). The other half of patients will be randomized to receive the active drug first. After a washout period of one week, they will receive the placebo prior to a second testing session (A/P group). As plasma levels of armodafinil peak between 2-4 hours after administration, participants will be asked to take a single 250mg capsule 2 hours prior to the scheduled testing sessions.