Some clinicians are still hesitant to use the diagnosis of borderline personality disorder (BPD) in adolescence whereas numerous studies have shown evidence to support its validity within this population. Yet, many epistemic issues remain. Indeed, the concept of BPD in adolescence has inherited the issues from the adult one, to which is added the complexity of this developmental period. Borderline disorder is a confusing and polymorphic clinical object with two different origins: one psychiatric with a nosographic approach and the other psychanalytical with a structural one. On the one hand, BPD in adolescence is considered as a psychiatric diagnosis based on the same criteria used in adult psychiatry, with many studies describing core symptoms at this period such as repetitive non-suicidal self-injury and suicidal behavior. On the other hand, BPD is also considered as one of many options, a behavioral one, to express an internal “borderline” structure. In line with this statement, the role of social and family factors is to be taken into account. Moreover, within a developmental approach, some authors considered difficult to diagnose a personality disorder before 18 years old, while others considered adolescence as a “borderline” period in itself. In this oral communication, we will try to approach these epistemic challenges raised by this phenomenon in the field of adolescent psychiatry and, in order to do so, we will use data from several qualitative studies.