Magnetic resonance imaging (MRI) can be helpful in securing the diagnosis of multiple sclerosis (MS) when findings are typical and have a characteristic interval evolution on subsequent MRI scans. Typical MRI changes seen in MS include ovoid, sub-centimeter, white matter T2 hyperintense lesions with little or no mass effect or edema, located in the intracranial, infratentorial, and spinal cord (especially cervical) areas. The first clinically apparent episode of MS is typically referred to as a clinically isolated syndrome (CIS). With the widespread use of MRI, occasionally patients are encountered who have an MRI highly suggestive of MS in the complete absence of a relevant clinical history or abnormal neurological examination. Some have designated this group as radiologically isolated syndrome (RIS). The timing of a follow-up MRI after the diagnosis of MS depends primarily on what clinical question is being asked. This chapter discusses MRI safety and magnetic field strength.