The Knowledge Is Power Program (KIPP) is a network of charter schools designed to improve the educational opportunities available to low-income families. KIPP schools seek to boost their students' academic achievement and ultimately prepare them to enroll and succeed in college. To achieve these objectives, KIPP schools leverage strong student-behavior policies with rewards and sanctions; contracts between students, parents, and teachers; longer school days and school on Saturdays; substantial autonomy for principals; and close monitoring of school performance in terms of student achievement and college readiness. Not everyone is convinced, however. Skeptics have asked whether attrition of students from KIPP schools may explain their apparent success and thereby raised doubts about the prospects for replicating that success at scale. In the study described, the authors detailed student-level data to compare patterns of entry, attrition, and replacement in 19 KIPP middle schools and in traditional public middle schools in the districts in which the KIPP schools are located. They found that, on average, KIPP middle schools admit students who are similar to those in other local schools, and patterns of student attrition are typically no different at KIPP than at nearby public middle schools. In both groups of schools, students who leave before completing middle school are substantially lower-achieving than those who remain. KIPP schools replace fewer of these students in the last two years of middle school, however, and, compared to district schools, KIPP schools tend to replace those who leave with higher-achieving students. The data available for this analysis, however, clearly show that KIPP's impacts cannot be explained by advantages in the prior achievement of KIPP students. Even when attrition and replacement throughout the middle school years are taken into account, the limited range of potential peer effects at KIPP schools does not explain the large cumulative impacts on student achievement identified by prior studies.