The history of the borderline concept is characterized by controversies, many of which are caused by the lack of generally accepted diagnostic criteria. In this study — by providing a survey of the relevant literature — attention is paid to the development of operational diagnostic criteria and to examination and interview techniques which try to specify and isolate a group or subgroups of so-called borderline patients. Recently some progress has been made in finding more reliable diagnostic criteria and methods. However, the validity of the concept still has to be established. The future determination of generally accepted reliable and valid diagnostic criteria and methods will be crucial to further development of the borderline concept.