Multiple sclerosis (MS) is an inflammatory demyelinating autoimmune disease of the central nervous system. The disorder displays marked clinical heterogeneity. In certain cases, making diagnosis can be challenging. Diagnosis of MS has become more important in the era of treatments that change the natural history of the disease. Several general diagnostic principles are useful to guide the diagnostic approach to MS. Clinically, MS requires neurological problems associated with objective abnormalities. Certain basic principles, first outlined by Schumacher et al. (1965) are still pertinent. Poser et al. (1983) have further modified the criteria using data derived from clinical evaluation and laboratory studies, including cerebrospinal fluid analysis, evoked potentials, and imaging studies. Poser criteria have long been familial for most neurologists. The most recent addition to our diagnostic armamentarium are the McDonald criteria (2001), which are the first attempt to incorporate standardized MRI criteria into the MS diagnostic process. The most innovative use of MRI to support an MS diagnosis is dissemination of demyelination can be demonstrated by MRI alone, in the absence of any new clinical attacks. Diagnosing MS by such sensitive MRI criteria will occur more quickly than waiting for a second clinical event. This has added some sensitivity, some controversy, and a lot of confusion. The application of the new criteria on Asian MS patients remains to be validated. Each of the criteria will be discussed, with major emphasis on the McDonald criteria.