The number of charter schools grew rapidly for a quarter-century after the first charter opened its doors in 1992. But since 2016, the rate of increase has slowed. Is the pause related to a decline in charter effectiveness? To find out, the authors tracked changes in student performance at charter and district schools on the National Assessment of Educational Progress, which tests reading and math skills of a nationally representative sample of students every other year. They focused on trends in student performance from 2005 through 2017 to get a sense of the direction in which the district and charter sectors are heading. They also controlled for differences in students' background characteristics. This is the first study to use this information to compare trend lines. Most prior research had compared the relative effectiveness of the charter and district sectors at a single point in time. The authors' analysis shows that student cohorts in the charter sector made greater gains from 2005 to 2017 than did cohorts in the district sector. The difference in the trends in the two sectors amounts to nearly an additional half-year's worth of learning. The biggest gains are for African Americans and for students of low socioeconomic status attending charter schools. When adjusted for changes in student background characteristics, they find that two-thirds of the relative gain in the charter sector cannot be explained by demography. In other words, the pace of change is more rapid either because the charter sector, relative to the district sector, is attracting a more proficient set of students in ways that cannot be detected by demographic characteristics, or because charter schools and their teachers are doing a better job of teaching students. Given the rising achievement levels at charter schools, the slowdown in the sector's growth rate cannot be attributed to declining quality. It is more likely that political resistance to charters is increasing as both the management and labor sides of the district sector become increasingly concerned that charters might prove to be as disruptive an innovation as the transistor.