Personality consists of enduring patterns of perceiving, relating to, and thinking about the environment and oneself that are exhibited in numerous social and personal contexts . A personality disorder arises when personality traits are so inflexible and maladaptive across a wide range of situations that they cause significant impairment of social, occupational, and role functioning or subjective distress. In addition, the thinking, displays of emotion, impulsivity, and interpersonal behavior of the individual must deviate markedly from the expectations of the individual's culture in order to qualify as a personality disorder. There are 10 personality disorders included in the American Psychiatric Association Diagnostic and Statistical Manual, Fourth Edition, Text Revision (DSM-IV-TR)  (table 1).
Borderline personality disorder (BPD) is characterized by instability of interpersonal relationships, self-image, and emotions and by impulsivity. The name of the disorder was originally based upon a concept that this syndrome constituted the boundary or borderline between neurosis (a pattern of maladaptive traits and coping mechanisms) and psychosis (a condition of grossly impaired reality testing) .
Patients with BPD receive extensive clinical attention and the disorder is more widely studied than any other personality disorder [3,4]. Despite these efforts, patients with BPD continue to suffer considerable morbidity and mortality.
The epidemiology, clinical features, diagnosis, and differential diagnosis of BPD will be reviewed here. The treatment and prognosis of BPD and other personality disorders are discussed separately. (See "Borderline personality disorder: Treatment and prognosis" and "Personality disorders".)
BPD is common in both the general population and in clinical settings. Large, nationally representative, nonclinical surveys of the US general population estimate that the point prevalence of BPD is 1.4 percent and the lifetime prevalence is 5.9 percent [5,6]. Studies in clinical settings found BPD was present in 6.4 percent of urban primary care patients, 9.3 percent of psychiatric outpatients, and about 20 percent of psychiatric inpatients [1,7,8].