Eighty percent of individuals with multiple sclerosis (MS) initially develop a clinical pattern with periodic relapses followed by remissions, called relapsing-remitting MS (RRMS). This period of fluctuating disease may last for a decade or more. Clinical relapses reflect acute inflammation in the central nervous system (CNS), composed of the brain and spinal cord. Often, different anatomic areas in the CNS are involved each time a relapse occurs, resulting in varied clinical manifestations in each instance. Relapses are nearly always followed by some degree of remission, though recovery to baseline status before the flare is often incomplete. There are nine approved drugs for treatment of RRMS. The most potent drug for inhibiting relapses, the humanized anti-α4 integrin antibody known as Natalizumab, blocks homing of mononuclear cells to the CNS. The mechanisms of action of the approved drugs for RRMS provide a strong foundation for understanding the pathobiology of the relapse. Despite substantial progress in controlling relapses with the current armamentarium of medications, there is much to learn and ever more effective and safe therapies to develop.