Teachers are learners of language teaching, which is a life-long and on-going process. Every teacher is responsible for her own development. Dedicated teachers learn throughout their careers, actively participating in a culture where they learn more about the content they teach and about the instructional and assessment practices they use with that content. To change their practice, teachers must go through a regular process of self-analysis, inquiry, and reflection. This book consists of a study conducted at a state university in Turkey to encourage professional development of English language instructors. Critical Friends Group (CFG) model was applied to promote teacher collaboration. Through a development-based in-service teacher training (INSET) programme, participant teachers in the School of Foreign Languages were expected to explore their own professional development, as well as to foster peer development by the help of CFG.