Teacher in-service learning about education reforms like NGSS often begin with professional development (PD) as a foundational component (Supovitz & Turner, 2000). Teacher Leaders, who are early implementers of education reform, are positioned to play a contributing role to the design of PD. As early implementers of reforms, Teacher Leaders are responsible for interpreting the purposes of reform, enacting reforms with fidelity to meet those intended goals, and are positioned to share their expertise with others. However, Teacher Leader knowledge is rarely accessed as a resource for the design of professional development programs. This study is unique in that I analyze the knowledge Teacher Leaders, who are positioned as developers of PD, bring to the design of PD around science education reform. I use the extended interconnected model of professional growth (Clarke & Hollingsworth, 2002; Coenders & Terlouw, 2015) to analyze the knowledge pathways Teacher Leaders' access as PD developers. I found that Teacher Leaders accessed knowledge pathways that cycled through their personal domain, domain of practice and domain of consequence. Additionally the findings indicated when Teacher Leaders did not have access to these knowledge domains they were unwilling to continue with PD design. These findings point to how Teacher Leaders prioritize their classroom experience to ground PD design and use their perceptions of student learning outcomes as an indicator of the success of the reform. Because professional development (PD) is viewed as an important resource for influencing teachers' knowledge and beliefs around the implementation of education reform efforts (Garet, et al., 2001; Suppovitz & Turner, 2000), I offer that Teacher Leaders, who are early implementers of reform measures, can contribute to the professional development system. The second part of this dissertation documents the instantiation of the knowledge of Teacher Leaders, who are positioned as designers and facilitators of PD. I examine the extent to which Teacher Leader knowledge is instantiated into specific resources and tasks during PD specifically for the Next Generation Science Standards (NGSS). The findings indicate that Teacher Leaders' knowledge is instantiated in tasks that promote and facilitate alignment of Teacher Leader goals for NGSS science practices-based instruction, which are framed around student learning outcomes. I offer a number of ways in which these findings can help educators and PD developers to better structure activities that present an alternative vision for science education that also provides the needed resources to shape how classroom tasks are designed and managed in ways that attend to and build on the practical knowledge of Teacher Leaders. The third part of this dissertation addresses the role Teacher Leaders play in this reform context with respect to their contributions to the professional development system. Based on the analyses of the Teacher Leaders in this study, I claim Teacher Leaders are essential contributors to the professional development system that extends beyond their typical role of participant. I argue that Teacher Leaders bring special expertise to the role of designers and facilitators of PD programs, and to the role of ambassadors for professional learning communities in a reform context. Because Teacher Leaders have a broader influence on the professional development system as pictured here, the Teacher Leaders in this study represent an essential piece of the reform puzzle. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]