Many elementary teachers enter the profession with anxieties about teaching mathematics that can impact their approaches to math instruction. Though teacher education programs often strive to alleviate these anxieties, the process of learning to teach math is complex, and novice teachers often enter the field with little support for continued learning. As such, it can become difficult for early-career teachers to continue to develop and sustain positive mathematics teaching identities towards responsive instructional practices. This three-part study seeks to understand how early-career teachers negotiate their own mathematics teaching identities in different organizational contexts and with different learning supports. This study contributes to the current body of research on early-career teachers' identity development as they enter the field by introducing a dynamic systems model of mathematics teaching identity and using this framework to explore ECTs' developmental trajectories in different school contexts and with different learning supports. Using multi-modal data collected from teachers in their first three years of teaching at different schools across the United States, this study applies comparative qualitative case studies along with Epistemic Network Analysis to understand the complex dimensions of early-career teachers' experiences teaching mathematics. Findings from this study offer insights into the design of professional learning structures and school-based organizational systems that support early-career teachers to enact high-quality mathematics instruction. [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.]