Studies of the charter sector typically compare charters and traditional public schools at a point in time. These comparisons are potentially misleading because many charter-related reforms require time to generate results. We study quality dynamics among charter schools in the State of Texas from 2001-2011. School quality in the charter sector was initially highly variable and on average lower than traditional public schools. However, exits, improvement of existing charter schools, and higher quality of new entrants increased charter effectiveness relative to traditional public schools despite an acceleration in the rate of sector expansion in the latter half of the decade. We present evidence that reduced student mobility and an increased share of charters adhering to No Excuses- style curricula contribute to these improvements. Although selection into charter schools becomes more favorable over time in terms of prior achievement and behavior, such compositional improvements appear to contribute little to the charter sector gains. Moreover, accounting for composition in terms of prior achievement and behavior has only a small effect on estimates of the higher average quality of No Excuses schools.