Teacher leaders are vital to establishing a collaborative school culture that fosters continuous improvement of teaching and student achievement. Over the last seven years, Learning Forward's support of teacher leaders has focused more on teachers who support their colleagues by serving in one or more of many roles with one of many diverse job titles. A fundamental belief underlying Learning Forward's work is the importance of collective responsibility. This belief acknowledges that no school or system will succeed based on the leadership of a single hero leader. Deep change requires that all parties within a school or school system work collaboratively and productively to realize results for every student. In "The Learning Educator: A New Era for Professional Learning," Stephanie Hirsh and the author (2007) describe eight principles that guide effective professional development. This article discusses three of these principles which serve as the foundation for teacher leadership: (1) leadership; (2) expertise; and (3) collaboration.