In this chapter, Lesson Study (LS) is introduced as a model of professional development, which provides teachers with an opportunity to research their own practice with a view to improving students’ learning. LS is growing in prominence worldwide, particularly in STEM education. In this research, a group of post-primary Mathematics teachers participated in LS during their implementation of a revised curriculum. Their work was analysed utilising a framework of Mathematical Knowledge for Teaching (Ball, Thames, & Phelps, 2008) and findings demonstrate a prominence of knowledge specific to teaching mathematical content (i.e. specialised and pedagogical content knowledge) employed by teachers during LS.