Progress in understanding the pathophysiology of MS has shown that irreversible damage to the central nervous system may occur early in the disease course. Evidence from clinical trials suggests that part of this damage might be prevented by the early use of disease modifying drugs; this therefore increases the importance of an early MS diagnosis. All diagnostic criteria for MS have incorporated definitions for dissemination in space and in time, along with the exclusion of alternative diagnoses. The McDonald criteria include magnetic resonance imaging definitions for such concepts, which enable MS to be diagnosed much sooner, in many cases. This paper reviews the steps to be taken in the diagnostic evaluation of patients with suspected MS.