As the commonest primary neurological disorder of young adults, especially in Europe and North America, multiple sclerosis is a public health problem of considerable significance for individuals and society alike. Its pathological basis has long been known, yet much of the etiology of the disease remains to be elucidated. Prevalence of multiple sclerosis varies markedly with geographical location, onset is generally insidious, and the course of the disease is largely unpredictable. While these characteristics, together with possible genetic and environmental risk factors, provide enormous scope for epidemiological studies of multiple sclerosis, they also pose appreciable methodological problems. The respected researchers Dr Sharon Warren and Dr Kenneth Warren have provided a comprehensive overview of this field of investigation. They deal with diagnostic criteria, methodological issues, results of worldwide studies, risk factors and susceptibility, and disease course and prognosis. Drawing together all of these considerations and summarizing both what is known and what remains to be discovered about multiple sclerosis, the authors conclude with insightful suggestions for future areas of research.