Over the past decade, school districts and states have made impressive advances in collecting and managing data used for accountability purposes. By the year 2011, all 50 states will have systems to track students from year to year. But in most states and districts, all the data generated by these systems flows only one way: up. This paper describes New York City's efforts to create an evidence-based and collaborative teaching culture. Few other districts have embraced the use of data like the nation's largest school system. As such, its experience offers valuable lessons for other schools and systems seeking to maximize the use of data to drive and inform classroom-level instruction: First, even if the technology works, it holds little value unless it is "flexible" and "relevant"--and it incorporates the sort of fine-grained information that teachers really want and need. Second, building a data system is only the first step--building the conditions and demand for data-based analysis is often more difficult than collecting the data itself. The country is now entering the second phase of a costly and concerted push to use data to improve educational outcomes. As this case study shows, the challenge is no longer whether to build institutional data systems, but how to use the data that the best of these systems provide to make a difference in the classroom and boost outcomes for every student. (Contains 4 figures and 55 notes.)