Over the past 25 years, the number of assistant principals has been steadily increasing, as has the number of principals with prior experience as an assistant principal. However, the knowledge base on assistant principals has not grown in parallel with their increased presence in schools. There is also little discussion about how the assistant principal role can promote equity and diversity in the pathway to the principalship as well as contribute to equitable experiences and outcomes for students, teachers, and staff. In this report, the authors present the results of a systematic synthesis of 79 empirical research studies on assistant principals published since 2000, including both quantitative and qualitative studies. To address gaps in this research base, the authors supplement the synthesis with new analyses of national data and data from two states, Tennessee and Pennsylvania. This report provides a descriptive portrait of the assistant principal role. It then addresses two important issues: diversity and equity among assistant principals and assistant principals' influence on student and school outcomes. The report addresses five research questions: (1) How prevalent are assistant principals? Which principals tend to have experience as an assistant principal?; (2) What are assistant principals' leadership roles?; (3) What is the pathway to the assistant principalship and from the assistant principalship to the principalship?; (4) How does access to the assistant principalship and principalship differ by educators' race, ethnicity, and gender?; and (5) What is the relationship between the assistant principal role and school outcomes? What is the relationship between experience as an assistant principal and future principal performance? Based upon the interpretation of the research, the authors set forth an agenda for redefining and reframing the assistant principal role in three areas. They also present an agenda for future research.