Multiple sclerosis (MS) is an immune-mediated inflammatory demyelinating disease of the central nervous system. There are neither diagnostic tests nor markers for MS. Other causes must be excluded thoroughly before an MS diagnosis is considered definitive. The relapsing-remitting type is a major clinical course in MS. Most disease-modifying drugs (DMDs) target this clinical type. Many clinical studies have demonstrated that early intervention with DMDs may prevent disease progression and transition from relapsing-remitting MS to the secondary progressive type. There are windows of opportunity for DMDs. An adequate DMD should be used for the appropriate patient at the optimal time.