Magnetic resonance imaging (MRI) continues to evolve, providing almost boundless information about tissue structure and function that can be obtained noninvasively in vivo. MRI is a key tool in the diagnosis and longitudinal monitoring of patients with multiple sclerosis (MS). The technique is highly sensitive for the definition of brain and spinal cord involvement in MS, including the ability to detect multifocal lesions, diffuse (occult) disease, and macroscopic atrophy. Conventional MRI techniques, which include T2-weighted, T1-weighted, and gadolinium-enhanced imaging, are used primarily to detect overt lesions but can also be used to quantify tissue atrophy. Variations of these techniques also are important, and refinements are under way. The ability to demonstrate lesion dissemination in space and time has led to the common use of MRI as a paraclinical measure to support a diagnosis of MS, including in patients with clinically isolated demyelinating syndromes. In addition, MRI is used to monitor the progress of disease in patients with clinically definite MS, including assessment of lesions and atrophy. The use of conventional MRI in the diagnosis and longitudinal management of patients with MS is the focus of this review.