This chapter summarizes interferon (IFN) biological effects, its possible mechanisms of action, and the key studies in clinically isolated syndromes (CIS), relapsing remitting multiple sclerosis (RRMS), and progressive MS. Measures of specific IFNβ-induced products, such as oligoadenylate synthetase (OAS), β-2 microglobulin, or neopterin, have been useful in pharmacodynamic studies to determine the magnitude and duration of the IFNβ-response, since serum levels of IFNβ are undetectable following injections. A consistent finding of follow-up studies from the three pivotal IFNβ RRMS trials and two CIS trials is that early treatment is beneficial compared with delayed treatment. IFNβ is partially effective in clinical trial groups. Since approval 18 years ago of the first IFNβ product for RRMS, treatment effects of β at all stages of MS have become fairly clear. The development of β for MS has illustrated many of the challenges in developing treatments for MS.