It was aimed to examine pre-service teachers' developing understandings of doing statistics within a lesson study. It was focused on their understandings (i.e., content knowledge) of doing statistics as well as how they transform their understandings (i.e., content knowledge) into teaching practices (i.e., knowledge of student & knowledge of teaching) while designing and implementing lessons related to graphs. Three senior pre-service teachers participated in a two phase (university and school classroom) lesson study. Data were collected through lesson plans prepared by the pre-service teachers in groups, observations, field notes, semi-structured interviews and video and audio recordings of the group meetings and instructional implementations. Results showed that they did not consider doing statistics as it consisted of many inter-related components initially. Their understandings developed greatly as they designed lesson plans, discussed the concepts and reflected on their lesson plans and instructional implementations. Their starting to put statistical questions into the center of doing statistics became an important turning point that affected their conceptions related to other themes (e.g., collecting data, interpreting graphs). It was also observed that their understandings related to doing statistics impacted their teaching practices.