As with other clinical phenomena, the historical analysis of the term, concepts and behaviours involved in the construction of 'depersonalization' should provide researchers with an essential frame for its empirical study, Before the term was coined in 1898, and under a variety of names, behaviours typical of 'depersonalization' were reported by Esquirol, Zeller, Billod, and Griesinger. The word dépersonnalisation derived from a usage in Amiel's Journal intime, was first used in a technical sense by Ludovic Dugas. The new disorder has since been explained as resulting from pathological changes in the sensory system, memory, affect, body image and self-experience. During the 1930s, evolutionary views became popular, particularly in the work of Mayer-Gross. The unclear conceptual boundaries of depersonalization still invite confusion and often enough fragments of what used to be its core-behaviour are used to diagnose the disorder. Depersonalization has of late become subsumed under the dissociative disorders. The definitional instability of the latter, however, has caused further complications to the study of depersonalization. It is recommended that the term is used to refer only to the original core-behaviour as this has shown adequate stability.