Multiple sclerosis (MS) is a chronic neurological disease, which is known to be more common in UK than many countries. While there are multiple genetic factors affecting risk of developing MS and disease severity within the condition, evidence on the rising worldwide prevalence and the increasing female to male preponderance has focused interest on environmental factors influencing MS risk. Data on several of these factors are reviewed, with particular focus on those such as vitamin intake, smoking and obesity, which may be influenced at a personal and population level by medical advice. When patients ask what has caused their illness, they are usually referring to the aetiology rather than the pathological basis. For example, people with multiple sclerosis (MS) asking about the cause of their condition are not interested so much in T-cell abnormalities and other immune dysfunction as knowing some of the factors that may have predisposed them to develop this autoimmune condition. The aim of this article is to review the data on MS risk factors, with particular emphasis on those which may be modifiable through medical advice at a personal and population level