The first problem in doing useful research on psychopathy, research that is replicable and can be aggregated with the research of others, is the problem of diagnosis, of identifying a group of probands who can be said to belong to the same relatively homogeneous species or psychopathological type. Hare and his colleagues in British Columbia, Megargee and Moffitt in the United States, and Blackburn in England have been important contributors to this effort and their accomplishments comprise the topic of this chapter.