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Interferon-beta (IFN-β) therapy has a central place in the management of multiple sclerosis (MS). The three recombinant IFN-β preparations currently available have shown benefit on activity measures (relapses and active lesions apparent on magnetic resonance imaging), while therapy advantages on progression measures (disability and total lesion burden) are less consistent. Moreover, IFN-β is effective only in a percentage of patients, since in many of them neutralizing anti-IFN-β antibodies develop after 6-18 months of treatment, leading to loss of drug bioactivity. Comparative data across studies made with different IFN-β preparations suggest that the optimal choice of IFN-β subtype, preparation and dose regimen are important determinants of efficacy. Because IFN-β actions depend on the activation of IFN-inducible genes, in addition to the direct quantification of anti-IFN-β antibodies, several other methods for the measure of IFN-β biologic activity have been recently developed. Among these, the determination of the IFN-β-inducible gene product Myxovirus protein A (MxA) has proven to be the most reliable one. Another still open point is the role of the differential expression of IFN-β receptor (IFNAR) components, since IFNAR2 subunit can be synthesized in three isoforms: functional, truncated non-functional and soluble. While this and other important issues require further studies, this article reviews and discusses the importance, potential and limits of the methods currently available to monitor IFN-β therapy in MS patients.