Multiple sclerosis (MS) affects approximately 2.5 million individuals worldwide. The prevalence of MS varies considerably across the world, the highest prevalence being in North America and Europe, and lowest in East Asia and sub-Saharan Africa. While the specific cause(s) of MS are largely unknown, they likely involve a complex interaction of genetic and environmental factors. Individuals with an affected first-degree relative have a 2–4% risk of developing MS, compared to a 0.1% risk in the general population. For most individuals, the major source of vitamin D is sunlight exposure. As duration and intensity of sunlight are strongly linked to latitude, the higher incidence of MS at higher latitudes has been proposed to reflect vitamin D deficiency. MS is characterised by the complex interplay of immunological and neurodegenerative processes, culminating in heterogeneity in the clinical course and symptom expression. Cortical pathology is a frequent phenomenon in MS, and observed in all stages of the disease.