Within the past several years, research on the clinical phenomena and neurobiology of borderline personality disorder has increased substantially. Borderline personality disorder is currently best thought of in terms of dimensions rather than as a categorical disorder. This article reviews the most recent findings on two of the core dimensions--affective dysregulation and impulsivity. Most of the neuropsychologic, physiologic, endocrinologic, and neuroimaging data support the theory that a dual brain pathology, affecting prefrontal and limbic circuits, may underlie this hyperarousal-dyscontrol syndrome.