The diagnosis of multiple sclerosis (MS) is based on demonstrating evidence of inflammatory-demyelinating injury within the central nervous system that is disseminated in both time and space. Diagnosis is made through a combination of the clinical history, neurologic examination, magnetic resonance imaging and the exclusion of other diagnostic possibilities. Other so-called "paraclinical" tests, including the examination of the cerebrospinal fluid, the recording of evoked potentials, urodynamic studies of bladder function, and ocular coherence tomography, may be helpful in establishing the diagnosis for individual patients, but are often unnecessary. Differential diagnosis in MS must be guided by clinical presentation and neurologic localization. While the list of conditions that can mimic MS clinically or radiologically is long, in clinical practice there are few conditions that truly mimic MS on both fronts. A positive test for a putative MS "mimic" does not unto itself exclude the diagnosis of MS. Typical symptoms of MS include discrete episodes ("attacks" or "relapses") of numbness, tingling, weakness, vision loss, gait impairment, incoordination, imbalance, and bladder dysfunction. In between attacks, patients tend to be stable, but may experience fatigue and heat sensitivity. Some MS patients go on to experience, or only experience, an insidious worsening of neurologic function and accumulation of disability ("progression") that is not associated with discrete relapse activity. Progression accounts for most of the long-term disability in MS. Diagnostic criteria for MS have evolved over the past several decades, with each revision impacting the apparent prevalence and prognosis of the disorder - the result has been to encourage earlier diagnosis without compromising accuracy.