Previously, research about teacher expertise has adhered to relatively fixed notions of teacher expertise. However, in this study, we share data from teacher study group (TSG) meetings, which demonstrate a dynamic understanding of teacher expertise. In these meetings, teachers discursively positioned themselves, their colleagues, and the research team as both experts and learners as they engaged in a community of enquiry around questions of curriculum design and implementation. We argue that teachers' dynamic and multiple positionings generated especially striking opportunities for learning. As teachers externalized their expert knowledge, their assumptions were brought forth to be examined, challenged, and reconsidered, thus opening space for further learning. We argue that these TSGs thus existed as a site for teacher learning, in which teacher dialog around curricular design, redesign, and the student learning that occurred in the lessons influenced their own learning about their students' capabilities. We make a case for further research that explores how teacher expertise is interwoven with episodes of teacher learning. We assert that a complex understanding of teacher expertise, grounded in principles of learning occurring through social interaction, has important implications for how the field should move forward in its approach to fostering teacher learning throughout teachers' careers.