Teaching Lab is a nonprofit organization that works to close the educational opportunity gap through teacher-led professional learning. We identify exceptional teacher-leaders, work with them to recruit their colleagues, and support them in improving instruction for under-served students. In order to raise the achievement of under-served students, we must help thousands of teachers improve their instruction. Despite US schools investing over $20 billion annually in teacher training, typical professional development (PD) simply does not work. Most PD is ineffective because it is characterized by low teacher buy-in, generic content, and lack of follow through. Teaching Lab offers an effective and scalable alternative for rapidly improving teaching quality. Teaching Lab uses a model based on three key elements supported by cognitive science and education research: Teacher Leadership, Core Content, and Iterative Cycles of Inquiry. These elements in combination create impact greater than the sum of their parts.