Teaching is a dynamic profession with many factors influencing change and growth. There are new standards every decade, new students every year, and new technology seemingly adopted before full integration of old technologies. Teachers must regularly develop to keep up with change. Unfortunately, when teachers reflect on traditional learning opportunities through school-sponsored professional development, only 25% report satisfaction with what their schools offer (Boston Consulting Group, 2014). Teachers cite dissatisfaction with both the insufficient time included for professional development during the school year, and also the misalignment of content in the professional development with their contextual needs. To combat inadequate development teachers are often looking outside of their school for learning opportunities. This study investigates the ways teachers expand their personal learning networks to meet their individual learning needs. While teacher learning opportunities are often guided by agreed upon design principles of professional development (Darling-Hammond, Hyler, & Gardner, 2017), teacher voice is rarely called upon in the design process. Despite evidence suggesting the positive impact of teacher-directed professional learning on student outcomes (Akiba & Liang, 2016), little is known about how teachers leverage their personal learning networks to meet their contextually relevant learning needs. To further our understanding of mathematics teachers' personal learning networks and the knowledge that is shared within these networks this study seeks to examine: a) the complexity of teacher personal learning networks, b) how teachers use their networks to develop professionally, and c) what knowledge is available to teachers as they expand their learning networks into informal online learning communities. This dissertation analyzed survey data to examine the complexity of teachers personal learning networks. Using a learning ecology survey design (Barron, 2004) the survey measured what teachers desired to learn about and whom they networked with to professionally development around their desired learning topics. The findings from this survey provide a detailed description of how teachers are both conceptualizing and navigating their personal learning networks to learning about the teaching and learning of mathematics. The survey also provided a detailed understanding of the complexity of teacher learning networks by quantifying the number of learning topics and learning partners teachers turn to for support. Teachers are relying on larger learning networks for a variety of learning topics that go beyond those topics and opportunities provided by traditional professional development. To further understand how teachers use their personal learning networks, 19 case study interviews were conducted. These interviews informed the creation of two teacher learner profiles. The profiles (Knowledge Broker, Lurking to Learn) illuminated how, and why, some mathematics teachers expand their learning networks to augment traditional professional development opportunities. Findings from these case study interviews suggest particular ways teachers are using their networks to support their own learning and also the learning of those they support in their face-to-face learning communities. Finally, this study examines the knowledge that teachers access within an online informal learning community. As teachers expand their personal learning networks into these communities they gain access to numerous new learning partners. These learning partners, often other classroom teachers, are providing support and knowledge around the teacher and learning of mathematics. [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.]