A review article on multiple sclerosis (MS) is difficult because the pace of current research in the evaluation and treatment of the disease has accelerated greatly in the past 5 years. The rapidly expanding field of immunology together with the plethora of new drug trials makes the task of including all relevant material impossible. This first of a two-part article will include several sections on MS, including an overview, pathophysiology, diagnostic evaluation, and clinical parameters. The second part, to appear in the December issue, will look at treatment strategies and future directions.