Procedure: Please see study overview in Appendix C. Subjects will be recruited through physician referrals from the MS Clinic at UMDNJ in Newark, NJ (see letter of support, Appendix D). All potential subjects will undergo initial neuropsychological (NP) testing (Appendix E) to screen for study inclusion criteria, namely the presence of new learning impairment. Subjects will only be enrolled in the current protocol if they are classified as impaired in new learning and memory ability, with intact language abilities. Such impairment will be defined as performance at least 1.5 standard deviations (SD) below the mean of a healthy control sample on the Open Trial Selective Reminding Test. This test has been shown to be sensitive to deficits in new learning abilities commonly observed in MS. In addition, performance on the Token Test (see Appendix D) must be within normal limits. All subjects qualifying for the study will then undergo baseline neuropsychological testing to document current levels of functioning in new learning and memory abilities. Subjects will then be randomly assigned to either group 1 or group 2. A two-period crossover design will be used in which subjects will be randomly assigned to one of the two groups. Subjects in both groups will be seen by a physician (Dr. Greenwald) who will assess their current neurological status and complete the EDSS. Group 1 (n=10) will first undergo treatment with Modafinil (200 mg once per day in the morning) for 2 weeks. They will then undergo follow-up neuropsychological assessment and follow-up EDSS to evaluate any medication effects. After the follow-up evaluation, there will be a washout period of one week in which no medication will be administered. Group 1 will then receive a placebo medication for 2 weeks. A second follow-up evaluation will be conducted following this latter arm of the study. Group 2 (n=10) will follow the same pattern, but will receive the placebo medication during the first arm of the study and Modafinil during the 2nd arm of the study. Importantly, the subjects and the experimenter administering the neuropsychological evaluation will be blind to group membership. All subjects will be asked to keep a precise journal during both arms of the study, recording the time at which the medication was taken, any side effects, activities during the day, and any changes in cognition for each activity. They will be encouraged to contact the prescribing physician if they encounter any difficulties. See Table 1 for study protocol. A power analysis is presented in Appendix F.