Although reflection has a key position in the development of teachers' pedagogical practices, few studies have investigated the development of physical education teachers' reflections over time. Against this backdrop, this study seeks to answer the following research question: "How does a one-year participatory action research project using Cooperative Learning as a pedagogical intervention influence PE teachers' reflections about teaching and learning?" The first author, Lars, assumed the role of researcher-facilitator, supporting the teachers' pedagogical development while simultaneously collecting data relevant to the research question. Interviews, observation notes and the researcher's reflective diary were analysed using an abductive approach. The study concludes that the interplay between: (a) new theoretical perspectives, (b) the establishment of a collectively reflective community and (c) the prolonged project duration eventually enabled the teachers to critically reflect upon their previous practices. At the same time, their journeys over the course of the project consisted of ups and downs and can be understood through three critical cycles: "establishing systematic, collective and cyclical processes," "new spaces of experience" and "the tipping point." Drawing upon our findings, we discuss various ideas for future educational action research projects aimed at challenging traditional practices within and beyond the PE context.