This study examined teachers' positioning of English language learners (ELLs) and instructional strategies to support them within the Problem Solving Cycle professional development program. Using a communities of practice lens (Wenger, 2000) and building on literature related to supporting ELLs in mathematics, Mathematics Knowledge for Teaching (MKT) (Ball & Bass, 2000), and professional development for teachers, I investigated how two emerging communities of practice, one composed of teacher-leaders and the other a school-based professional development that a teacher-leader led. Using ethnographic microanalysis of interaction (Erickson, 1992), participants' discussions across eight meetings of instructional leaders, four meetings school-based professional development, and multiple interviews were analyzed. The participants within the communities of practice assisted in determining which topics would be discussed and whether supporting ELLs was a priority in their planning and enactment of mathematics problems. In the community of practice of teacher leaders, there was less of a focus on ELLs as not all teachers recognized that they had ELLs. In these instances, students were "invisible" ELLs, because of this lack of recognition. Conversely, within the school-based community of practice, situated in a school with a higher proportion of ELLs, the teachers spoke often about how they could support these students. The Problem Solving Cycle provided structural supports for teachers' development of intended classroom practices and for their roles as leaders within a professional development setting. The findings of this study highlight two key themes. The first is related to the role of emerging communities of practice. In these communities, facilitators created opportunities for teachers to discuss ELLs, to reflect on their teaching practice, and to develop as mathematics educators and leaders. The second theme is related to the strategies for supporting ELLs that the participants developed. Teachers' and facilitators' positioned ELL strategies as supportive of all learners, thereby ensuring that strategies to support ELLs could be part of any teachers' practice, even if they did not believe they had ELLs. The teachers in the school-based professional identified specific mathematics knowledge associated with using these ELL strategies, while considering the benefits and constraints of using these particular strategies. [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/en-US/products/dissertations/individuals.shtml.]