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The etiology of multiple sclerosis (MS) remains unclear. However, as with almost all common complex traits, genetic and environmental components have important roles, both independently and interactively, in disease susceptibility. Stochastic and epigenetic effects cannot be overlooked [1–3] – see Figure 16.1. In MS, much of the work on molecular genetics (e.g. linkage studies, candidate genes, genome wide association studies (GWAS)) [4] and genetic epidemiology (“a science which deals with the aetiology, distribution, and control of disease in groups of relatives and with inherited causes of disease in populations” [2,5]) has focused on adults. Nevertheless, it is reasonable to expect, at least to some extent, overlap with the pediatric MS population, as defined in this textbook by Chabas and Waubant. This is not to say that pediatric-onset MS cases have been specifically excluded from molecular genetic studies. Rather, in most instances, all ascertained patients have been included, but identification has been through adult-orientated neurologists and clinics [6–8]. With the increasing recognition of pediatric MS as a distinct entity [9,10], it is now becoming very clear that genetic/genetic epidemiological data must be compared and contrasted for the following groups.