Multiple sclerosis is a chronic demyelinating disease of the central nervous system that occurs primarily in young adults. There is no single diagnostic test to recognize the disease. The diagnostic criteria, based on clinical examination and laboratory tests, have changed considerably over time. The first guidelines involved only the results of the patient's neurological examination. The diagnostic criteria developed by Poser in 1983 were based largely on the results of additional tests, including visual evoked potentials and analysis of cerebrospinal fluid. The McDonald criteria, developed in 2001 and updated in 2005 and 2010, reflected the diagnostic breakthrough caused by widespread use of magnetic resonance imaging (MRI). Currently, the diagnosis depends largely on the results of the MRI examination. An early diagnosis is particularly important for starting disease-modifying treatments.