The abstract nature of mathematics requires the communication of mathematical ideas through multiple representations, such as words, symbols, pictures, objects, or actions. Building representational fluency involves using mathematical representations flexibly and being able to interpret and translate among these different models and mathematical concepts. This article presents a collaborative lesson study experience in planning and teaching a unit on decimals. Participants included fifth- and sixth-grade teachers and lesson study facilitators, including a university mathematics educator, a doctoral student, and a school mathematics specialist. The goal of the lesson study was to develop students' representational fluency and mathematical proficiency with decimals. Here, the authors describe the three phases involved in the lesson study process: (1) the collaborative planning phase; (2) the teaching and observation phase; and (3) the debriefing phase. Some common themes that emerged in the conversation regarding representations are summarized. (Contains 4 figures.)