This chapter describes the prevalence of mood disorders in the context of borderline personality disorder (BPD). Atypical depression is characterized by mood reactivity in the context of a depressive episode and at least one of the following symptoms: increased sleep, increased appetite, leaden paralysis, and rejection sensitivity. Suicide attempters with co-occurring BPD and major depressive disorder (MDD) have more lifetime suicide attempts, make their first attempt at a younger age, report more interpersonal triggers to attempts, and have higher levels of life-time aggressive behaviors, hostility, and impulsivity than depressed attempters without BPD. A number of studies have reported altered brain structure and function in patients with BPD relative to controls. Although the brain imaging findings in BPD may lead to diagnostic biomarkers, many of the altered brain regions and systems associated with BPD overlap with mood disorders and other Axis I disorders.