In this dissertation, I K-8 mathematics teacher agency and learning within a research-practice partnership (RPP) context. I asked three primary research questions: (a) How is teacher agency enacted within the RPP?; (b) what do teachers perceive they are learning from their participation in the RPP?; and (c) how is teacher agency connected to teacher learning in the RPP context? I utilize both an ecological framework for teacher agency (Priestley et al., 2015a) and situated learning theory (Lave & Wenger, 1991) about teacher learning to frame the questions and analyses. As an independent investigation that drew from a larger, multi-site project (i.e., as part of the broader RPP), case study methodology (Yin, 2018) was employed to closely examine K-8 mathematics teachers' participation and experience in a co-design team in one school district. The team's primary goal was to improve the quality of mathematics instruction across their school district by employing a design-based implementation research (DBIR) process. The district and school administrators and teachers were involved in the improvement team's work. Multiple types of data were collected including semi-structured interviews, meeting observations, and related documents for two years. Additionally, those data sources were supplemented with a targeted set of semi-structured interviews about teacher agency and teacher learning to better address the study's research questions. I analyzed the data using primarily qualitative methods and, specifically, with coding approaches associated with grounded theory (Bernard et al., 2016; Charmaz, 2006). The study's findings suggested teachers enacted their agency in various ways in and outside of their team meetings and the quality of engagement changed over time. From the ecological perspective of teacher agency, multiple mediating factors from iterative, projective, and practical-evaluative dimensions were found to influence teachers' participation by functioning as supporting and inhibiting factors. Amid the teachers' involvement in the RPP, teacher learning occurred not just about mathematics teaching but also about other areas, such as leadership, collaborative abilities, and understanding of the DBIR process, which are all crucial components of capacity building for creating sustainable learning opportunities even after the partnership with the university. Also, the interviews revealed potential connections between teacher agency and learning. In the final chapter, I delve into the implications of these findings. [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.]