This qualitative study seeks to establish a deeper understanding of how and what teachers and teacher educators learn collaboratively during the lesson study process in a Singapore primary school. We used the boundary theories to conceptualize this learning process and delineate the learning mechanisms to foster mutual learning between the teacher educators and teachers in the case school. It was found that the teachers' "practical" concerns and the "improvement" proposals from the teacher educators were constantly being negotiated considering the perceived and received "consequences," which drove the boundary actions that include both "boundary making" and "boundary crossing" to form a learning space for the participants. Findings from this study provide a practical perspective that explains the complexities, challenges, and possibilities of implementing lesson study and working with boundaries to support teacher professional learning.