Teacher leaders who share their specialized knowledge, expertise, and experience with other teachers broaden and sustain school and classroom improvement efforts. Teacher leaders can transform classrooms into learning laboratories where every student is engaged in relevant and well-designed curricular content, every teacher embraces the use of more effective instructional strategies, and authentic assessments provide evidence of rich student learning. This work describes four essentialities associated with teacher leaders: a focus on student learning, along with the importance of empowerment, relationships, and collaboration. In addition to gleaning insights from the literature, examples of the impact of teacher leaders in schools are provided to demonstrate the importance of each.