This thesis investigates the inter-rater reliability of the Structured Interview for DSM-IV Personality (SIDP-IV), a semi-structured diagnostic interview for assessing DSM-IV personality disorders, in an outpatient population of adolescents between the ages of 16 and 18. The 25 participants in the study were interviewed and evaluated for the presence of personality disorders by experienced clinicians, and audio recordings were made from these sessions. Based on these recordings I blindly re-coded the interviews. This data was used to calculate the inter-rater reliability in the form of the intra-class correlation coefficient (ICC) and Cohen’s kappa coefficient. These results obtained indicated high inter-rater reliability for the instrument, with an ICC coefficient for criterion scores of 0.952 and a kappa for dichotomous diagnostic assessment of 0.896. The data analysed was collected and made available by the research group behind the First Experimental Study of Transference In Teenagers (FEST-IT), who have graciously provided supervision and support throughout my work.