A growing body of research points to the ways in which principals influence teachers, classrooms, and, ultimately, student achievement. New Leaders aims to prepare transformational school leaders by partnering with districts and charter schools to offer rigorous, research-based training for aspiring principals. The Aspiring Principals program is New Leaders' signature program and has three core features: selective recruitment and admission, training and endorsement, and support for principals early in their tenure. This report is a follow-up to the 2014 evaluation of New Leaders' Aspiring Principals program. Focusing on the revised program, which was first implemented in 2012, the authors present evidence of the effectiveness of the revised Aspiring Principals program and share lessons that can inform principal-preparation policy and practice. To assess the effect of New Leaders' Aspiring Principals program, researchers analyzed whether schools and students led by graduates of the program outperformed comparison schools and students in the same district, focusing on student achievement and principal retention. They also examined program graduate placement and satisfaction with the Aspiring Principals program. [For the appendixes, see ED605724.]