Uhthoff's phenomenon (also known as Uhthoff sign or Uhthoff syndrome) is described as temporary, short-lived (less than 24 hours) and stereotyped worsening of neurological function among multiple sclerosis patients in response to increases in core body temperature. This phenomenon is named after Wilhelm Uhthoff, a German ophthalmologist who described it. In 1890, Uhthoff first described exercise-induced amblyopia in multiple sclerosis patients. In 1961, this phenomenon was given his surname, Uhthoff's Phenomenon (UP), by G. Ricklefs[1]. In four out of 100 MS patients, Uhthoff observed the appearance of reversible optic symptoms induced by an increase in body temperature, "marked deterioration of visual acuity during physical exercise and exhausting"[2]. Subsequent observations have shown that the same physiological mechanism responsible for visual dysfunction in the setting of heat exposure, are also responsible for a variety of other neurological symptoms experienced by MS patients. When Uhthoff studied this phenomenon, exercise was thought to be the etiology, and the significance of elevation in body temperature escaped his notice. Six decades later in 1950, the hot bath test was developed based on this phenomenon and was used as a diagnostic test for multiple sclerosis. By 1980, with advancement in neuroimaging, hot bath test began to be replaced by other diagnostic tests such as MRI and cerebrospinal fluid analysis because of its unspecific nature and potential complications. The temporary worsening of neurological function in response to heat exposure affects the physical and cognitive function of multiple sclerosis patients and interfere with their activities of daily life and functional capacity. This worsening needs to be differentiated from a true relapse or exacerbation of MS. An understanding of this phenomenon and its pathophysiology, therefore, is essential for recognition and appropriate treatment.