The purpose of this study was to understand the perceptions of in-service Physical Education (PE) teachers when using Teaching Games for Understanding (TGfU) in teaching sports. Data were gathered from interviews, work group meetings, and participants' diaries. The results show the difficulties PE teachers had in the planning and implementation of TGfU. In the initial stage of implementing TGfU, teachers reported feelings of insecurity to the point of doubting their own pedagogical expertise and knowledge. They also reported anxiety and exhaustion. Once they surpassed the first stage, teachers' feelings of satisfaction increased in parallel with students' improvement, in particular because students with the lowest skill level had made significant progress in decision-making, overall compression of the game, and tactical problem solving. This study identified some major challenges facing PE teachers wishing to implement TGfU, and thus allows for the development of support strategies to promote teachers' pedagogical self-assessment.