Teacher leadership is widely thought to be critical to the success of local school improvement efforts. But it is often unclear what teacher leadership entails, precisely. Supported by district-level staff, and with assistance from a university-based research team (the authors of this article), a group of science teacher leaders created a conceptual framework to guide their work. Teacher leadership, they concluded, involves four main activities: collaborating with teachers, modeling effective instruction, providing resources, and advocating on teachers' behalf.