The Assessment of Identity Development in Adolescence (AIDA) is a recently developed measure of identity diffusion in young people. Originally validated in Switzerland, the English version has not yet been validated. Our aim was to evaluate the AIDA's internal factor structure, internal consistency, incremental validity, and relations to other variables in adolescents and young adults and derive clinical cutoffs for use in clinical settings. Study 1, involving 2,119 undergraduate students between the ages of 18-25 years, confirmed the expected bi-factor structure for the AIDA. Study 2 involved the recruitment of 122 adolescent inpatients (ages 12-17), of whom 36 met full DSM-based criteria for Borderline Personality Disorder, in addition to 164 adolescents of the same age, recruited from public schools (total 286 adolescents). Results of both studies demonstrated strong internal consistency and correlations of the expected magnitude and direction with other self-report measures of personality pathology. In addition, Study 2 demonstrated that the AIDA could successfully discriminate between adolescent inpatients with and without BPD, as well as healthy controls, and established preliminary clinical cutoffs that should be replicated in future studies. In all, these studies provide support for further validation and use of the AIDA in young people.