Studies of the charter school sector typically focus on head-to-head comparisons of charter and traditional schools at a point in time. The impact of the charter reform on the overall quality of schooling, however, depends crucially on whether introducing a charter sector improves the quality of schooling options, particularly in high-poverty communities. We study changes in the quality of charter schools in Texas between 2001 and 2011. Our results suggest that the creation of the charter sector initially led to the entry of a set of schools that were highly variable in terms of quality and less effective on average than traditional public schools. However, exits from the sector, improvements of existing charter schools, and positive selection of charter management organizations that open additional schools raised the average charter school effectiveness over time relative to traditional public schools in Texas. Although data limitations preclude the identification of the specific causes of charter school improvement, the evidence is consistent with the belief that more effective schools have greater success in attracting and retaining less disruptive and higher-achieving students. It also appears that reductions over time in teacher and student turnover have contributed to the improvement in the charter sector.