PSYCHOANALYTIC theory would suggest that differentiating the borderline adolescent from his or her normal peers should be a difficult task. In many respects the developmental crises of adolescence dovetail with critical conflicts of Borderline Personality Disorder--e.g., identity formation and separation-individuation. Extensive semi-structured interviews (Gunderson's Diagnostic Interview for Borderlines) of normal, borderline and other disturbed adolescents provide a data base for examining the hallmarks of borderline pathology in adolescence. This paper focuses on the qualitative differences in patterns of impulsivity, affective lability, dissociative experience, and interpersonal relationships that distinguish borderline teenagers from other seriously disturbed and normal adolescents. The paper also outlines modifications in the Gunderson interview for an adolescent population.