This chapter discusses how current operational definitions can apply to clinical practice and research settings of pediatric multiple sclerosis (MS) and acute disseminated encephalomyelitis (ADEM). It emphasizes strengths and limitations of these clinical practice and research settings. ADEM requires the presence of both encephalopathy and polysymptomatic presentation. Three major controversies concerning the criteria of dissemination in time relate to the fact that a consensus has not been reached about when to call pediatric MS a recurrent disease in: patients with a first ADEM-like episode who further develop non- ADEM-like episodes, patients with recurrent ADEM including more than two episodes, and patients with recurrent non-ADEM-like events such as optic neuritis (ON) or transverse myelitis (TM) without brain MRI findings and without neuromyelitis optica (NMO) IgG. The first two situations are fairly specific of the pediatric population, given the higher frequency of ADEM or ADEM-like presentations in children, especially before puberty.