Data-driven instruction began its spread across the country about a decade ago, in the footsteps of the No Child Left Behind requirement that schools administer yearly achievement tests. Those tests didn't help teachers spot and backfill learning gaps, though. Scores came back after everyone had moved on to the next grade, and anyway, the tests were designed to hold schools accountable for the performance of groups: Did enough English-learners pass, enough African Americans? They were not intended to show which students didn't understand decimals. By most accounts, a few charter schools began testing their youngsters more frequently, with the idea that teachers could use those interim results to inform their teaching. But on the ground, data-driven instruction has encountered problems. Schools complain that interim assessments produced by publishers aren't always aligned with curricula, pacing guides, or year-end state tests. The assessments are often too easy, handing schools an unhappy surprise when state test results are posted. Some districts have taken over the job of producing interim tests, but their data offices have the reputation of taking so long to return results that the information is too old to be of much use. Schools that have tried to develop their own assessments have found the job overwhelming. In any event, few teacher-education schools include data-analysis training, so many teachers don't know how to read the data, or don't have the time to use the information to rethink their lesson plans. By the mid-2000s, "data was starting to become a hot topic." But "teachers were saying they wanted help" understanding and using it. Achievement Network (ANet) is a nonprofit organization that combines rigorous, standards-aligned assessments; data-analysis training and coaching for school leaders and teachers; guided peer review; and networking across schools. Schools join ANet, pay a fee for its services, and commit their teachers and principals to a four-times-a-year cycle of testing and data review. The model goes beyond traditional professional-development models by linking ANet's work to each school's data feedback loop: student achievement results inform the guidance ANet provides.