The classification of personality disorders (PDs) is in transition. The shortcomings of the categorical PD diagnoses led to the development of the Alternative Model for Personality Disorders (AMPD) in Section III of the DSM-5 regarding a dimensional differentiation of severity of personality pathology. Impairments in personality functioning are defined as a general Criterion A for PD. Inspired by the AMPD, the LoPF-Q 12-18 was developed to assess Levels of Personality Functioning (LoPF) dimensionally in adolescents from 12 to 18 years old in self-report. The questionnaire shows good scale reliabilities, good construct validity by demonstrating a systematic variation with pathology in line with theory, and a valuable clinical utility. The results suggest that the LoPF-Q 12-18 questionnaire is a clinically useful instrument to assess personality pathology in young people and that the concept of personality functioning is useful in detecting and describing central impairments of PD pathology at an early stage of development.