Plagued by long-term poor student outcomes, a number of reforms have been implemented within the Philadelphia School District to improve performance, including the use of charter schools. The number of charter schools in Philadelphia has risen from four to more than 60 over the past decade, and these schools now serve over 30,000 students. Debate surrounding these schools continues to spark intense debate. Critics charge that charter schools siphon off the best students from the traditional public schools, divert badly needed resources from the school district, and foster racial enclaves within a district. Supporters defend the charter schools, claiming that they not only improve the performance of their students but also that the competitive pressures charters generate cause the traditional public schools to improve. For this study, researchers collected a longitudinal database from the Philadelphia school district for school years 2000-01 through 2006-07. This database enabled researchers to track student movement between schools and provides student-level information of race/ethnicity, grade, and test scores in math and reading. Findings reported include: (1) Charter schools are attracting students whose prior achievement levels (when they were in traditional district schools) are slightly below the district-wide average, but higher than the average achievement levels of the traditional public schools they left; (2) Students are transferring to charter schools with a slightly larger population of their own race/ethnicity than the traditional public schools from which they come; (3) Using a fixed-effect model to examine whether students who switch between traditional public schools and charter schools have stronger math and reading gains in charter schools, analysis indicates that students' average gains attending charter schools are statistically indistinguishable from the gains they experience while at traditional public schools; (4) Performance of Philadelphia's charter schools as measured by student achievement gains does not appear to be related to how long the charter school has been operating, although attending a charter school in grades 9-12 shows a small positive effect on student achievement while attending a charter school in elementary and middle grades shows a small negative effect; (5) No evidence was found to indicate that district schools located in neighborhoods with the greatest charter competition are performing any better or any worse as a result of the competition; and (6) Charter schools and traditional schools have similar percentages of students who change schools from year to year. The authors caution that reported findings are not intended as fully comprehensive, and advise that policymakers and citizens take into account other factors in assessing the value of charter schools for the city, including, but not limited to, student attendance, graduation, college attendance, disciplinary rates, program and course offerings, and cost effectiveness. Four appendixes include: (1) Categories of Charter Schools Based on Mission Statements; (2) Regression Results; (3) Racial/Ethnic Makeup of "Switching" Students Relative to All Charter Students; and (4) Details for Calculating Mobility Rates. (Contains 31 footnotes, 1 figure and 13 tables.) [Additional support provided by the Stranahan Foundation.]