Improving the teacher workforce is now high on the education policy agenda, and most policymakers draw on research to inform their positions about teacher quality. Many policymakers cite the "fact" that, in general, teachers don't improve their effectiveness after their first five years in the classroom. However, recent research shows that the reality is more nuanced, and that many teachers do show improvement in their teaching ability throughout their careers. The authors present findings from their recent study showing that, over 10 years, the test scores of students of particular teachers increased, indicating that these teachers' skills were increasing throughout this 10-year period. Papay and Kraft also found that teachers vary a lot in how much they improve over time, and that teachers in some schools improved more, implying that school conditions play a role in promoting or constraining teacher growth. The authors discuss, and provide examples of, four levers schools or districts can use to promote long-term teacher improvement: peer collaboration, teacher evaluation, tailored on-the-job training, and organizational supports.