It is well reported in the literature that professional development activities have great potential to increase teachers' competencies in different areas, resulting in greater learning opportunities for students. In Turkey, however, teachers' participation in professional development activities is significantly lower compared to almost all developed countries. In this context, this study aims to explore the different teacher- and school-level factors associated with teachers' participation in professional development activities in Turkey by using a nationally representative data set from the Teaching and Learning International Survey (TALIS) and employing a multi-level statistical analysis with Hierarchical Linear Modeling (HLM). Results of the study show that several teacher- and school-level factors are significantly associated with teachers' level of participation in professional development activities in Turkey. Specifically, the significant role of school principals in teachers' professional development is explored. It is therefore suggested that school principals should be educated on the importance of in-service professional development activities for teachers and their significant role in this matter. (Contains 2 tables and 1 footnote.)