This article focuses on 3 conceptual challenges that we sought to address while conducting a design experiment in which we supported the learning of a group of middle school mathematics teachers. These challenges involved (a) situating teachers' activity in the institutional setting of the schools and district in which they worked, (b) developing an interpretive framework that enabled us to document the collective learning of teacher groups, and (c) reconceptualizing the relations between teachers' activity in professional development sessions and in their classrooms. For each challenge, we describe both the initial approach that we developed while preparing for the experiment and the modifications that we found it necessary to make once the experiment was in progress. Our goal in sharing these reflections from the field is to contribute to the task of extending the design experiment methodology to investigate the learning of groups of practicing teachers rather than groups of students. (Contains 9 footnotes.)