This qualitative phenomenological study aims to explore teachers' perceptions of professional learning communities and how they feel it impacts their effectiveness as secondary mathematics teachers in two instructional domains--content knowledge and pedagogy. Teachers spend a substantial amount of their time engaged in Professional Learning Communities. This study explored how teachers perceived the PLCs they engage in within their schools. It examined if the model they follow in their schools impacted teacher instructional practices, which led to student achievement. This study is imperative because the data collected provides information on how PLCs, if implemented correctly, can be conducive to teachers' success. According to Richard DuFour (2004), the impact of professional learning communities can have a long-lasting impact but states the term "professional learning community has been used so ubiquitously that it is in danger of losing all meaning" (DuFour, 2004, p. 6). The following three research questions guided this study: What are secondary mathematics teachers' experiences engaging in professional learning communities? What do secondary mathematics teachers believe they gain from time in professional learning communities? How do secondary mathematics teachers believe their instructional practices have changed because of participation in a PLC? This study was grounded on the Constructivist Learning Theory focused on the principle that learning is a social activity. The research participants included secondary mathematics teachers in the U.S. The findings in this study have implications for district and school leaders to play an essential role in ensuring that the development and sustainability of Professional Learning Communities in all schools are conducive to building teacher capacity in secondary 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.]