In this essay, the authors share a professional dialogue about the ways in which issues of power emerge in preschool classrooms when teachers endeavor to build on children's home and school mathematical experiences and understanding. From different perspectives, as early childhood and mathematics education researchers, the authors discuss ways in which data from teacher interviews and discussions collected during a professional development program provide evidence of whose knowledge is privileged. The authors use the dialogue to explore "what," "how," and "who" pre-K teachers most often privileged in their work with children and families in mathematics. And what effect that privileging had on power relationships.