Quality Professional Development (PD) interventions are important to improve teaching in schools (Desimone, 2009) and students' achievement (Wayne, Yoon, Zhu, Cronen, & Garet, 2008). Recent research has been focusing on learning using a situative perspective (Greeno, 2006). Within this body of research, this study analyzed elementary school mathematics teachers' learning during a PD intervention and examined the role of PD materials during the intervention. The PD intervention was conceptualized as a boundary encounter, and the PD materials as boundary objects (Wenger, 1998). The research is comprised of a one-case study (Creswell, 2013), bounded by a cohort of teachers participating in a PD intervention. The study followed an embedded mixed-methods design. The study characterized main patterns in teachers' participation in the PD, and identified four shifts in their participation. It also showed that PD materials played an important role in the way teachers participated in the PD and supported the shifts in teachers' participation. Results from the study added to the research community's knowledge base about PD features that are important for teachers' learning (Desimone, 2009; Garet et al., 2001; Penuel, Fishman, Yamaguchi, & Gallagher, 2007) and contributed to research design and PD curriculum development. [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.]