This study investigates: (1) how principals' data-driven practices may vary by principals' and school backgrounds and how that changes over time; (2) how principals' data-driven practices influence teacher buy-in; and (3) how principals' data-driven practices and teachers' buy-in influence student outcomes. This study uses data from the Study of Instructional Improvement (SII). More-experienced principals tend to be less data-driven than less-experienced principals. The more likely principals are to use data in their practices, the more teachers buy into the programs, and experienced principals tend to improve school-level teacher buy-in. Collective teacher buy-in improves over three years and is positively associated with students' reading achievement.