In this article, we describe the conceptualization and development of the Secondary Transition Fidelity Assessment (STFA), a measure intended for use by high school leadership teams to examine secondary programs and practices demonstrated by research to lead to meaningful outcomes for secondary students with and without disabilities. We describe our conceptualization and development efforts, which resulted in a measure with six critical features: adolescent engagement, district-level capacity, school-level capacity, community engagement, family engagement, and professional capacity. Based on stakeholder feedback, we revised the STFA and developed an instruction manual and action planning guide to support the self-evaluation of secondary programs and practices using data from the STFA. Recommendations for the field related to research, policy, and practice are provided.