Borderline personality disorder is common in treatment settings and may be so in the general population. In this guide to assessment strategies for diagnosing borderline personality disorder, the authors discuss the reliability and validity of structured. interviews and self-report instruments and suggest the use of a self-report questionnaire as a cost-effective screening test. Assessment problems, such as the need for longitudinal observation, are reviewed. Essential, features of the recommended diagnostic approach include clarity about the diagnostic concept, consideration of the full range of diagnostic criteria, incorporation of recently developed diagnostic methodologies, care in distinguishing personality disorders from comorbid axis I syndromes, and complete assess. ment of the full range of axis II disorders.