In recent years, some of the most ambitious charter school operators, with the support of philanthropic investors, have sought to increase the scale and scope of their work by creating charter management organizations (CMOs) that aim to replicate effective charter school models across multiple campuses. CMOs are nonprofit organizations with unified management teams that have operational responsibility for delivering the educational program and supervising the school leaders for groups of charter schools. CMOs are now sufficiently mature to permit a systematic evaluation of their effectiveness. In New York City, the authors have access to a comprehensive data set that provides an opportunity to examine the impact of two established CMOs, Achievement First and Uncommon Schools (referred to hereafter as AF and Uncommon, respectively). In this report, the authors present findings from a nonexperimental analysis in which they estimate the effect of five New York City middle schools affiliated with AF and Uncommon on the achievement of their students. This report draws on the data collected for the National Study of CMO Effectiveness, which is being conducted by Mathematica Policy Research and the University of Washington's Center for Reinventing Public Education. The focus of this analysis is the achievement impacts of New York City middle schools operated by AF and Uncommon. The five schools examined here were included because they (1) were in a jurisdiction (New York City) in which the authors had data available; (2) are schools for which achievement data can be examined prior to entry (in third and fourth grades) and for each year of enrollment; and (3) have been operating long enough to include at least one year of outcome data for at least one cohort of students. The estimated impacts cover school years from 2005-2006 through 2007-2008. The authors compare alternate methodological approaches to ensure the robustness of estimated impacts. The analyses they present in this report include adjustments to minimize potential biases caused by student selection, grade repetition, and attrition during middle school. Appendices include: (1) Baseline Test Scores and Demographic Characteristics; and (2) Two Alternate Grade Repetition Models. (Contains 7 tables and 10 footnotes.)