Principals are essential to improving student achievement and narrowing persistent achievement gaps between students in underserved communities and their economically advantaged peers. The Every Student Succeeds Act provides opportunities for states to invest in developing and supporting effective school leaders. One such opportunity includes using federal funds to support the recruitment, preparation, and training of high-quality leaders using the optional state set-aside under Title II. To inform state investments, we summarize the evidence about the importance of principals and describe research-based practices in leadership development. Then we outline promising, evidence-based investments in school leadership from submitted and draft ESSA state plans, including states taking advantage of the optional leadership set-aside. Ensuring that all students are in a school with a quality principal requires states to adopt research-based investments that are implemented with fidelity.