The diagnosis of borderline personality disorder (BPD) can be devastating. BPD is characterized by instability on several domains: affect regulation, impulse control, interpersonal relationships, and self-image, and it affects about 1–2% of the general population—up to 10% of psychiatric outpatients, and 20% of inpatients. In addition to meeting the criteria set forth in DSM-5, BPD, like all personality disorders, is characterized by a pervasive and persistent pattern of behavior that begins in early childhood and is stable across contexts. Affective dysregulation (inappropriate, intense anger or difficulty controlling anger; affective instability due to a marked reactivity of mood), is one of the core domains associated with BPD and is characterized by erratic, easily aroused mood changes and disproportionate emotional responses. Affect dysregulation differs in BPD and mood disorders because in BPD it can shift rapidly and is affected by environmental triggers.