The dominant approach to second language (L2) teacher education emphasizes reflection as a tool for helping teachers develop context-specific, personal theories of L2 teaching. Educators can facilitate reflection by involving teachers in action research. This small-scale study investigated whether carrying out action research as part of a graduate seminar affected the professional development of graduate teaching assistants (TAs) who were teaching in foreign and second language departments. Insights into the TAs' professional development were gained through a qualitative analysis of their professional journals, reflective essays, action research reports, and oral and written feedback. The findings indicated that the TAs gained a broader understanding of research, developed an appreciation for peer collaboration, and adopted new L2 teaching practices. Suggestions for L2 teacher educators with an interest in incorporating action research into their graduate degree programs are offered.