The Knowledge Is Power Program (KIPP) is a rapidly expanding network of public charter schools whose mission is to improve the education of low-income children. As of the 2012-2013 school year, 125 KIPP schools are in operation in 20 different states and the District of Columbia (DC). Ultimately, KIPP's goal is to prepare students to enroll and succeed in college. Prior research has suggested that KIPP schools have positive impacts on student achievement, but most of the studies have included only a few KIPP schools or have had methodological limitations. This is the second report of a national evaluation of KIPP middle schools being conducted by Mathematica Policy Research. The evaluation uses experimental and quasi-experimental methods to produce rigorous and comprehensive evidence on the effects of KIPP middle schools across the country. The study's first report, released in 2010, described strong positive achievement impacts in math and reading for the 22 KIPP middle schools for which data were available at the time. For this phase of the study, the authors nearly doubled the size of the sample, to 43 KIPP middle schools, including all KIPP middle schools that were open at the start of the study in 2010 for which they were able to acquire relevant data from local districts or states. The average impact of KIPP on student achievement is positive, statistically significant, and educationally substantial. KIPP impact estimates are consistently positive across the four academic subjects examined, in each of the first four years after enrollment in a KIPP school, and for all measurable student subgroups. A large majority of the individual KIPP schools in the study show positive impacts on student achievement as measured by scores on state-mandated assessments. KIPP produces similar positive impacts on the norm-referenced test, which includes items assessing higher-order thinking. Estimated impacts on measures of student attitudes and behavior are less frequently positive, but the authors found evidence that KIPP leads students to spend significantly more time on homework, and that KIPP increases levels of student and parent satisfaction with school. On the negative side, the findings suggest that enrollment in a KIPP school leads to an increase in the likelihood that students report engaging in undesirable behavior such as lying to or arguing with parents. These findings are described in this report. The following appendixes are included: (1) Sample selection and baseline characteristics; (2) Constructing survey outcomes; (3) Schools attended by lottery winners and lottery non-winners; (4) Analytic methods for the matched comparison group analysis; (5) Analytic methods for lottery-based analysis; and (6) Validation of matching methods using lottery-based impact estimates. (Contains 46 tables, 78 footnotes, and 16 figures.) [For "What Works Clearinghouse Quick Review: 'KIPP Middle Schools: Impacts on Achievement and Other Outcomes, Final Report,'" see ED540896.]