NGSS Early Implementers is a six-year initiative created to help eight California school districts and two charter management organizations, supported by WestEd's K-12 Alliance, implement the Next Generation Science Standards (NGSS). The Initiative has used a train-the-trainer model to maximize spread of professional learning about the NGSS in each participating district. However, they expanded upon this model by explicitly preparing teachers to not only provide leadership in how to implement NGSS teaching, but also to become catalysts for change in their districts. This evaluation report explains how the Initiative prepared teachers for leadership in NGSS teaching, including how it created a culture of collaboration that produced change agents for science education and NGSS implementation. It also conveys how the leadership experience affected teacher leaders' actions and professional growth. Intended for school, state, and district leaders, the report addresses the following: (1) The organizational structure of teacher leadership in the Initiative; (2) How teachers were prepared for leadership in NGSS teaching; (3) How the Initiative empowered and challenged teachers to be change agents in their districts; (4) The many ways that Early Implementer teachers have provided leadership in NGSS implementation; and (5) The impact of the Initiative on teacher professional growth. Finally, recommendations are provided to administrators for encouraging and leveraging teacher leadership in support of NGSS implementation.