According to the alternative model for personality disorders (AMPD) of the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), a moderate or greater impairment in personality functioning is the essential criterion for a personality disorder diagnosis. Personality functioning is operationalized in the Level of Personality Functioning Scale via 4 domains (identity, self-direction, empathy, and intimacy) and 2 higher order dimensions (self and interpersonal functioning). The current study examined the reliability (interrater, test-retest), structure, and validity (convergent, discriminant, and incremental) of the Structured Clinical Interview for the AMPD-Module I (SCID-5-AMPD-I). A clinical sample (n = 121) completed the SCID-5-AMPD-I, along with an interview for DSM-5 Section II personality disorders and self-reports for personality pathology (personality functioning, personality organization, personality structure, and pathological personality traits) and other forms of psychopathology (depression, anxiety, somatization, and general disability). Interrater and test-retest reliability was excellent for overall personality functioning, the higher order dimensions, and the domains, except for the empathy domain in the test-retest condition. Factor analyses suggest that personality functioning is an essentially unidimensional construct. Personality functioning demonstrated high convergence with other forms of personality pathology and showed good discriminant validity in relation to depression, anxiety, and somatization but not in relation to the broader construct of general disability. Personality functioning (Criterion A) showed incremental validity over pathological personality traits (Criterion B) in predicting interview-assessed DSM-5 Section II personality disorders but not in predicting self-reported personality and general psychopathology. The present study suggests that the SCID-5-AMPD-I is a viable measure for personality functioning. (PsycInfo Database Record (c) 2023 APA, all rights reserved).