Although pronunciation instruction (PI) has started to gain prominence in second language teacher professional development (PD) research (e.g. Nguyen & Newton, 2021), relatively less attention has been given to PD programs in an online format. Adopting a Sociocultural Theoretical (SCT) perspective, this dissertation explored the role of an online pronunciation-specific PD program in mediating and facilitating the cognitive development and teaching practices of a cohort of in-service EFL teachers. Based on the communicative framework (Celce-Murcia et al., 2010), a 2-week online PD program was designed and implemented. Data were collected using varied sources: classroom observations, a background questionnaire, semi-structured interviews, narrative frames, stimulated recalls, and teacher-created e-portfolios. Classroom observations were transcribed and coded for pronunciation-related episodes. These episodes were quantitatively analyzed for duration, target of instruction, and instruction techniques utilized. Data collected through interviews, reflection journals, stimulated recalls, and e-portfolios were qualitatively coded and analyzed to explore how the teachers progressed through the PD program. Quantitative and qualitative data were triangulated using a microgenetic analysis to trace the development of teachers' cognitions and practices before, during, and after the PD program. Findings revealed that teachers' cognitions seemed to have developed and qualitatively shifted as a result of participating in the PD program. One notable development was the shift in their ideal goal of PI from focusing on nativelikeness to prioritizing intelligibility. In addition, the PD program enhanced teachers' content knowledge and pedagogical content knowledge and improved their confidence in teaching pronunciation. These developments, however, were not reflected in the classroom practices of half the teachers. Several institutional and sociocultural factors were identified that may have both influenced and limited the changes in teachers' practices, including the school's book-driven approach, students' expectations, and time constraints. The findings provide insight into the complex nature of teacher cognition development and its reflection in classroom practices. The study has several implications for teacher educators and materials developers. Since many in-service teachers lack adequate PI training, teacher educators should offer a pronunciation-specific module in their regular PD programs placing PI at equal value to other language skills. Additionally, given the teachers' reliance on textbooks, it is crucial that materials developers incorporate appropriate pronunciation lessons in textbooks. [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.]