The diagnosis of multiple sclerosis (MS) and other demyelinating diseases of the central nervous system is challenging, and although the currently available biological and imaging tools offer considerable support to physicians, these tools often fail to provide a simple and final answer at the time of a first event. Thus, sets of diagnostic criteria have been published and tested on patient cohorts, and are now used in clinical trials and in daily clinical practice. These criteria have evolved over time to take into account physicians’ and patients’ needs, along with emerging paraclinical tests. The different presentations of MS have given rise to the use of a common classification system to identify patient profiles and adapt care protocols accordingly. This article reviews the various classifications of the forms and diagnostic criteria of MS and related syndromes, including neuromyelitis optica (NMO)/NMO spectrum disorders (NMOSDs), acute disseminated (demyelinating) encephalomyelitis (ADEM) and chronic lymphocytic inflammation with pontine perivascular enhancement responsive to steroids (CLIPPERS). Also discussed is their validity in the light of the currently available literature.