Research across the decades has confirmed that effective school leadership is associated with better outcomes for students and schools. A high-quality school leader affects dozens of teachers and hundreds or thousands of students. It is a pivotal role. From 2011 to 2016, The Wallace Foundation, through its Principal Pipeline Initiative (PPI), provided funding and technical assistance to support six large school districts in their efforts to put in place systematic processes for the strategic management of school leaders. The purpose was to examine whether a comprehensive principal pipeline would be more effective than business-as-usual approaches to the preparation and management of school leaders. The term principal pipeline is shorthand for the range of talent management activities that fall within a school district's scope of responsibility when it comes to school leaders, including leader standards, preservice preparation opportunities for assistant principals and principals, selective hiring and placement, and on-the-job induction, evaluation, and support. This report documents what the PPI districts were able to accomplish, describing the implementation of the PPI and its effects on student achievement, other school outcomes, and principal retention. The authors found that all six PPI districts were able to implement comprehensive pipelines, and they did so in different ways. The PPI had positive effects on a wide range of outcomes that school districts care about, and evidence of these positive effects was widespread. The authors also found that the work is affordable: It cost a PPI district about $42 per pupil per year, or less than 0.5 percent of the district's budget in each school year, to operate and enhance its principal pipeline. The authors conclude that districts looking for ways to enhance school outcomes and improve the retention of newly placed principals should be encouraged by the experiences of PPI districts. The findings of this study suggest that when districts focused attention on activities related to principal pipelines, then principals, schools, and students benefited. Key findings include: (1) The work is feasible: (a) PPI districts were able to implement all components of a principal pipeline at scale; (b) PPI districts approached pipeline enhancement in different ways depending on their starting point, needs, and opportunities; (2) The work is effective: (a) After three or more years, schools with newly placed principals in PPI districts outperformed comparison schools with newly placed principals by 6.22 percentile points in reading and 2.87 percentile points in math; (b) Newly placed principals in PPI districts were 5.8 percentage points more likely to remain in their school for at least two years and 7.8 percentage points more likely to remain in their school for at least three years than newly placed principals in comparison schools; (c) The PPI had statistically significant, positive effects on achievement in elementary and middle schools and some evidence of positive effects for high schools; (d) PPI effects on achievement were positive and statistically significant for schools in the lowest quartile of the achievement distribution; and (e) PPI effects were positive and statistically significant in reading for five PPI districts and in mathematics for three districts. The PPI effect was negative and statistically significant in mathematics in one district; (3) The work is affordable: PPI districts spent about $42 per student per year on pipeline activities during the initiative. The lowest-cost components were the development of leader standards and selective hiring and placement; and (4) What drove these effects: The entire package of PPI components appears to have worked as a cohesive whole, much as it was designed to do. There was little evidence that individual components were uniquely correlated with larger or smaller effect sizes.