NCL nomenclature underwent a major change in 1969 (Zeman and Dyken, 1969) when the name ‘neuronal ceroid lipofuscinosis’ was used for the first time. This was proposed 25 years before the first NCL genes were identified. Since then the genes underlying most, but not all, human NCL disease have been identified, and it is clear that this group of diseases shows genetic heterogeneity (mutations in different genes may result in similar clinical disease phenotypes), allelic heterogeneity (different mutations within the same disease gene may result in very different clinical disease phenotypes), and can be phenotypically heterogeneous even within the same family (the same mutation in the same disease gene is linked with differing clinical symptoms and rates of disease progression). It is timely to review the current nomenclature, assess its accuracy and appropriateness, and, if necessary, revise it. Nomenclature is closely related to the classification of NCL diseases and has implications for NCL diagnosis.This chapter, and the following related Chapter 3, was initially drafted by the editors, who represent clinical, molecular genetic, biological, and morphological interests, and further revised by a panel of world experts in the NCLs.The overriding reason for classification of a disease is to provide a definition for subtypes that is universally understood. But first, a definition for the NCLs is required. A general definition of an NCL disorder is:A progressive degenerative disease of the brain and, in most cases, the retina, in association with intracellular storage of material that is morphologically characterized as ceroid lipofuscin or similar to it.