This document summarizes research on effective professional development for teachers. It finds that the most impactful professional development is collaborative, sustained over time, grounded in proven teaching practices, and focused on improving student outcomes. Two high-impact methods discussed are lesson study, where teachers collaboratively plan, observe, and reflect on lessons, and instructional coaching, where teachers receive support in implementing new strategies. The document advocates for moving away from one-off workshops and toward job-embedded professional learning done in collaboration with colleagues.