The purpose of this action research was to work with Brazilian trained educators in a Community of Practice (CoP) to explore how teachers collectively define and talk about critical thinking (CT). The research also examined how past teaching experiences shaped their attitudes toward emphasizing CT in teaching. In addition, the research studied how participation in a CoP focused on CT changed classroom planning. The study is grounded in Community of Practice and Social Constructivism. As an international school, this study examined related research conducted in Jordan, Hong Kong, Taiwan, and Pakistan. This qualitative action research was 12 weeks in length with six participants who were all from Brazil and worked at a private bilingual international secondary school. Participants completed an initial interview and final interview. They also completed online journals, which were assembled weekly for 45 minutes, and maximized their efforts constructing a unit plan utilizing the Understanding by Design method. The results of the study describe the teachers’ definition of critical thinking, and also present an understanding of how the CoP shaped their attitudes. This, in turn, resulted in members’ updated classroom planning, which was due to participation in the cohort. Further issues and credibility, contextualization, and transferability as well as researcher positionality were discussed.