Criterion A, as represented by Level of Personality Functioning (LPF), offers a means by which to conceptualize the core impairment in self and interpersonal functioning that distinguishes personality disorder (PD) from other forms of psychopathology. One of the most widely cited criticisms of the current Section II in Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition categorical diagnostic system is the high level of comorbidity among the categorical PD diagnoses. The Section III alternative Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition model for PDs (AMPD) addresses this problem by assessing the commonalities of the PDs using a single severity continuum, represented in the AMPD by the Level of Personality Functioning Scale (LPFS), which comprises both self and interpersonal pathology. Since its initial introduction, the LPFS has generated an expansive literature base and has also seen significant advancements in its measurement, including both semistructured and self-report methods. Such studies have generally demonstrated that the LPFS can be rated reliably, across both experienced and inexperienced raters, and demonstrates strong validity as evidenced by associations with relevant criterion variables and demonstrations of predictive and clinical utility. This review offers a brief history of the conceptualization and development of Criterion A and the LPFS, describes the major advancements in its measurement, and provides a summary of the status of the empirical literature regarding Criterion A's structure, reliability, and validity. (PsycInfo Database Record (c) 2022 APA, all rights reserved).