In reviewing the evidence for the validity of the diagnosis borderline, four descriptions in the literature seem to offer comprehensive criteria for the diagnosis. When the four are compared, a total of 104 criteria are enumerated encompassing the mental status, history, interpersonal relationships, defense mechansisms, and other judgments of personality functioning of the borderline patient. Half of these criteria are mentioned in only one of the four diagnostic descriptions. This apparent lack of agreement over diagnostic criteria has three possible interpretations: (1) the borderline concept is an illusion; or (2) the concept is adequately defined by those criteria held in common, the others being nonessential; or (3) apart from the concept defined by the common criteria, there are subtypes emphasized by different authors. Although we favor the third interpretation, it is suggested that further speculation await an adequate test of existing diagnostic criteria.