Many patients with multiple sclerosis (MS) experience clinical relapses or progression of disability, or exhibit evidence of disease activity on MRI, despite the use of disease-modifying therapy. Although evidence clearly supports the efficacy of interferon β (IFN-β) in treating MS, the factors that determine the response to this drug in individual patients have not been fully elucidated. As more treatment options become available, the early identification of factors that can affect or predict the efficacy of agents in individual patients is important, because such knowledge facilitates early switching of treatment. Despite years of research and numerous reports of promising therapy markers for MS, few markers have emerged as clinically useful. Several studies suggest, however, that development of MRI lesions within 6-24 months after the initiation of IFN-β treatment predicts an unfavorable response. In addition, persistently high titers of neutralizing antibodies diminish or abrogate the therapeutic effects of IFN-β, and help to identify patients who do not respond. This Review highlights advances in research on the response to IFN-β in patients with MS and aims to provide a practical approach for incorporating clinical data, biological markers and MRI measures of disease activity into their therapeutic management.