Journal of Social and Clinical Psychology 30(5): 441-457 Major Depressive Disorder (MDD) and Borderline Personality Disorder (BPD) are both significant mental health conditions that impact millions of individuals and have deleterious effects. There is a high rate of co-morbidity between these two disorders; in particular individuals with BPD report high levels of MDD symptoms. Identification of shared and specific vulnerabilities, both in terms of trait and emotion regulation variables, is an important step in explaining this overlap and developing interventions that address symptoms common or specific to these disorders. Dimensions of affect (i.e., negative affect, positive affect, affect intensity), avoidant emotion regulation (i.e., experiential avoidance, thought suppression, expressive suppression, nonacceptance), and symptoms of MDD and BPD were assessed in an unselected college sample. Results indicated that negative affect was related to both MDD and BPD symptoms whereas positive affect was only related to MDD symptoms and affect intensity was only related to BPD symptoms. Additionally, avoidant emotion regulation partially mediated the relation between dimensions of affect and symptoms. Implications and future directions are discussed.