In this article, we offer guiding principles for the assessment of personality disorders (PDs) in adolescents based on research on the development, manifestations, and impact of PDs in youth. First and most important, remember to assess PDs in youth, rather than assessing only non-PD conditions. Second, consider using the DSM-5 Alternative Dimensional Model of PDs. Third, assess acute symptoms and long-term underlying personality patterns and impairment. Fourth, evaluate past and present contextual factors (families, peers, schools, neighborhoods, SES, and culture). Fifth, gather information from both the adolescents themselves and knowledgeable informants. Sixth, focus assessment on problematic patterns of behavior, thinking, and feeling because those patterns will provide better targets for treatment than specific PD diagnoses. Seventh, highlight personality strengths and resources for change.