A growing body of information suggests that core or underlying personality is a significant concomitant of depression and suicidality. Introversion (ie, low extroversion) is especially promising in its relationship to the phenomenology and outcome of depression, and may represent an underlying heritable trait of etiologic significance. Furthermore, the presence of introversion has implications for differentiating unipolar and bipolar depression. It is likely that introversion acts in concert with other core personality variables, including neuroticism and having a feeling-type personality to influence depression. Considering depression from the perspective of core personality allows for novel psychotherapeutic approaches based on targeting underlying personality variables.