Purpose: This study investigated the impact of a Critical Friends Group (CFG), an inquiry-based professional development model, on Turkish EFL teachers. It was acknowledged that short-term workshops did not provide opportunities for teachers to make connections between the theory presented and the implications that it had for classroom teaching. CFG model accompanied with protocols was thought to be a practical and efficient way of professional development. Method: The research study was designed as a qualitative case study conducted at a Turkish state university and was aimed at exploring the impact of CFG as a tool to support professional teacher development. The methodology used was an inductive process using grounded theory. Data came from the journals, meeting transcripts, interviews and questionnaires, and from the researcher's notes. Findings: The results of the study showed that teachers who worked in a CFG felt better prepared to continue engaging in their profession. CFG provided the opportunity to work collaboratively, to delve into classroom-based dilemmas, to focus on the teaching and learning of specific academic content, and build strong working relationships among teachers. Implications for Research and Practice: CFGs are proved to be a valuable professional development model as teachers are given opportunities to take the time to inquire into areas of their teaching that they believe need attention. The collaboration should be formalized by school administrations by providing time and space for these processes to take place. Further studies should be implemented to observe the effects and impacts of CFG on students' learning.