Science teacher leaders are classroom teachers who take on leadership responsibilities within their schools, districts, and broader science communities. They have a significant impact beyond the science classroom by promoting instructional leadership, implementing science-related reforms, enhancing teacher professionalism, maximizing teacher effectiveness, and advocating for science. Gaps persist in understanding the development of science teachers into teacher leaders. A specific focus is needed on understanding the development of emergent science teacher leaders, who are experienced teachers without formal training, in teacher leadership programs. Areas that need further clarity are the development of leadership identity in teacher leadership programs, the elements from teacher leadership programs that are retained in the teacher leaders' schools after the conclusion of the program, and the interplay of factors that influence changes in leadership behaviors over time. This dissertation, comprising three studies, examines the identity development and leadership trajectory of emergent science teacher leaders through qualitative analysis. The first study analyzes how previous science teacher leadership programs supported a leadership identity that transformed science teachers into teacher leaders. The second study aims to discover specific factors or experiences from the teacher leadership program that are present in the identity of the science teacher leaders' post-program. The third study explores how personal attributes, environmental circumstances, and behavioral dynamics interact to influence changes in leadership behaviors over five years post-program. Across the three studies, three main themes emerge that influence the development of science teacher leaders pre- and post-program: (1) redefining leadership (an informal perspective on leadership, characterized by a partnership dynamic void of formal structure or authority), (2) interpersonal efficacy (the ability to engage effectively with others), and (3) collaborative community (supportive interactions that foster awareness, personal connections, diverse viewpoints, mutual engagement, and leadership growth). The findings from this study have theoretical and practical implications for science teacher leaders, schools, and leadership development programs. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]