Borderline personality disorder can be assessed in adolescence mainly through a dimensional approach that takes into account strengths and weaknesses of emerging personality patterns. Clinical interviews could help clinicians to gather information about adolescent’s functioning, fostering therapeutic alliance and promoting a mentalizing stance during the assessment of borderline adolescents. In this article, the Interview of Personality Organization Processes in Adolescence (IPOP-A) is presented through clinical case material to show its usefulness in clinical and research settings. The IPOP-A assesses an adolescent’s emerging personality along three dimensions: identity, quality of object relations and affect regulation. These dimensions are the core of borderline personality in adolescence.