Borderline personality disorder (BPD) is very common, with an estimated community prevalence of 1-3%, rising to 25% in psychiatric inpatients. Nevertheless, diagnostic assessment and disclosure in patients suspected to suffer from BPD can be difficult and even uncomfortable to many clinicians. In a survey among psychiatrists, 57% indicated they had failed to disclose a diagnosis of BPD at some point in their careers, citing diagnostic uncertainty and concerns about stigma as key issues (Sisti et al. 2016). This workshop will engage the audience in an intensive discussion of the challenges clinicians face when diagnosing borderline personality disorder (BPD). We will first review when and how to disclose a suspected diagnosis of BPD to a patient, and how to involve the patient in the diagnostic process. Secondly, we will work through some common difficulties during BPD diagnostic assessment, including the differential diagnosis with other personality disorders, trauma-related disorders and developmental disorders, as well as how to deal with comorbid psychiatric and somatic conditions. Furthermore, we will address how the new dimensional approach to the classification of personality disorders adopted by ICD-11 (WHO, 2018) diverges from the classical categorical case definitions used in the ICD-10 and DSM-IV/DSM-5 diagnostic frameworks, thereby significantly altering the concept of personality disorders. We will demonstrate how to diagnose BPD according to the new classification models, using the DSM-5 checklist and PID-5 diagnostic interview. Finally, we will illustrate some of the complexities of ICD-11 versus DSM-5 diagnostic perspectives with reference to a real-life court case.