This research report from Thomas B. Fordham Institute examines environmental factors and strategies that strengthen the professional development and retention of teachers, specifically those who work in high-poverty schools. Challenging the common misconception that teacher effectiveness remains stagnant after the first few years of teaching, the report finds that teachers continue to improve gradually when in supportive and collaborative work environments. It offers a comprehensive review of current and past research on the relationship between teaching experience and student achievement, the conditions for teacher improvement, and factors related to teacher retention. Lastly, the report concludes with four recommendations to support teacher growth: 1) make high-quality preparation and mentoring affordable and accessible, 2) provide quality professional learning opportunities, 3) prepare strong and collaborative school administrators, and 4) develop equitable policies to support teacher retention in high-poverty schools. Education leaders may find this report useful for informing discussions about the retention and ongoing development of experienced teachers.