The status of borderline personality disorder (BPD) as a diagnostic category is a matter of continuing controversy. In the United States, BPD is one of the most frequent diagnoses of psychiatric inpatients, and a similar tendency emerges in Europe. Nearly all theoretical aspects of BPD have been questioned, including its very position as a personality disorder. In this article, we trace the evolution of the borderline concept from the beginning of the 20th century to the current psychometric research. We argue that the status of BPD is fraught with conceptual difficulties, including an unrecognized semantic drift of major phenomenological terms (e.g., identity), a lack of general principles for the distinction of BPD and the major psychiatric syndromes (e.g., schizophrenia spectrum disorders), and insufficient definitions of key nosological concepts. These difficulties illustrate general problems in today's psychiatry that require consideration.