This review summarizes the current body of knowledge about borderline personality disorder (BPD). Each section describes the development of BPD’s knowledge base in the 33 years since the diagnosis became part of the American Psychiatric Association’s standard diagnostic classification system. The changes in our understanding are remarkable with respect to virtually every consideration of BPD’s etiology, course, and treatment. This expansion of knowledge has forced the authors to have to choose what we deem is most important, and thereby this review may overlook or underrepresent other significant developments. Nonetheless, we expect readers will share our excitement about what’s been learned. Throughout our review we will highlight very interesting and distinctly different questions that now surround this diagnosis.