Although patients with borderline personality disorder (BPD) are commonly seen in psychiatric practice, there has been far less biological research in BPD than in other psychiatric disorders. This article reviews the neurobiological research that has been performed to date in BPD and integrates the biological, psychological, and clinical findings in this disorder. BPD is best thought of in terms of dimensions rather than as a specific disorder. Each dimension has a biological profile and may be expressed differently in different patients. Four core elements are suggested to play a major role in the development of BPD: interpersonal stress, affective instability, impulsivity, and dissociation and self-injurious behavior. Genetic and environmental factors lead to brain alterations that are the basis for specific presentations of the disorder, such as self-injurious and impulsive aggressive behavior.