Federal mandates, such as, No Child Left Behind (NCLB) set high standards, but in reality did little to promote critical thinking instruction and learning in our nation's schools. Race to the Top is our nation's current attempt to improve education and thanks to this legislation there is now a set of common core standards aimed at infusing critical thinking into the curriculum. Districts in Arizona are struggling to provide common core training to prepare teachers to teach these new, rigorous standards. This is a problem because teaching critical thinking is challenging. While grade level teams often get together, little time is devoted to create lessons that are focused on deep learning and little time is set aside to observe lessons and reflect on student engagement. One potential solution to this may be lesson study. Lesson study is a method of professional development that encourages teachers to reflect on their teaching through a cycle of collaborative lesson planning and observation. The lesson study cycle connects with the constructed nature of learning provided by Vygotsky Space. This action research was designed to explore how 10 fourth, fifth, and sixth grade teachers at a K-8 school in Arizona learned how to infuse critical thinking into their lessons. This study took place from July to November of 2011. A mixed methods approach was used to collect data. Quantitative measures included Likert-items on a survey and lesson plans scored with the district rubric. Qualitative measures included open-ended survey items, transcriptions of lesson debriefs, reflective learning logs, and the researcher's personal field notes. Data were analyzed separately and then triangulated to reduce bias. Findings from this study indicate that although it was challenging for the teachers, lesson study enabled them to successfully integrate critical thinking into their lesson plans. The process of lesson study increased the teachers' efficacy to create lessons, and it helped them understand how important critical thinking was for their students. The teachers also came to value the lesson study process as a positive approach to professional development. Based on these findings, implications are made, and further action research cycles suggested.