The inclusion of maladaptive trait assessment in diagnostic processes of severe mental health problems in youth is increasingly valued. Early identification efforts in this regard are particularly oriented towards vulnerable youth with a developmental phenotype that resembles borderline personality pathology (BPP), known for its destructive outcomes when left untreated. The current project aims to elaborate the promising situational judgement strategy to comprehensively capture how these personality vulnerabilities are actually manifested in the daily life of youth, in response to validated situational triggers for youth BPP. This rationale can be understood from the evidence that vulnerable traits are often activated by perceptions of threat or adversity (i.e. trait-activation theory), leading to severe acting out behavior. In doing so, the current project will transfer a large body of evidence in support of this situation-based personality assessment from the area of applied psychology towards the field of clinical psychology. Overall, the research objectives aim to contribute to a more precise evaluation of situational trait dynamics and their relevance for clinical outcome. This effort should result in a concise strategy to systematically evaluate situation-based mechanisms of early BPP traits and should lead to concrete guidelines for targeting increased person-environment fit in vulnerable youth, which will eventually lead to more psychological growth.