Multiple sclerosis (MS) is one of the most common chronic neurological diseases in young adults in western countries. An important aspect of treatment of this disease is the use of interferons (IFNs). These are molecules with antiviral, immunomodulatory, antiproliferative and hormonal activities. IFNβ, a class I IFN, has been used extensively in the therapy of MS, particularly in its relapsing-remitting (RRMS) phase, the most frequent clinical form of the disease.Although the available evidence from published clinical trials is difficult to evaluate because of methodological differences, an unbiased review of the data reveals sufficient evidence to conclude that treatment with IFNβ in RRMS is both efficacious and safe, at least over the periods so far investigated (up to 4–6 years). While there is no reason to suspect that IFNβ should not continue to be efficacious and safe over the longer term, studies investigating these questions over longer periods and including greater numbers of patients are needed.