Background/Context This study builds upon research on teacher professional communities and high school restructuring reforms. It employs a conceptual framework that draws upon theories of “community of practice” and “community of learners.” Purpose/Objective/Research Question/Focus of Study This study analyzes how teachers’ professional inquiry communities at the high school level constitute a resource for school reform and instructional improvement. Setting This research focused on a reforming, comprehensive urban public high school with site-based management. Population/Participants/Subjects This study investigates the practices of six school-based oral inquiry groups known as Critical Friends Groups (CFGs), which were selected as cases of mature professional communities. Twenty-five teachers and administrators participated as informants. Research Design This research involved a video-based, qualitative case study. Data Collection and Analysis Data included observations of CFG meetings, interviews with teachers and administrators, and document collection. Analysis entailed coding with qualitative software, development of analytic cross-CFG meta-matrices, discourse analytic techniques, and joint viewing of video records with informants. Findings/Results The author explores four particular design features of CFGs—their diverse menu of activities, their decentralized structure, their interdisciplinary membership, and their reliance on structured conversation tools called “protocols”—showing how these features carry within them endemic tensions that compel these professional communities to negotiate a complicated set of professional development choices. Conclusions/Recommendations The findings demonstrate how the enactment of design choices holds particular consequences for the nature and quality of teacher learning and school improvement. Although CFGs enhanced teachers’ collegial relationships, their awareness of research-based practices and reforms, their schoolwide knowledge, and their capacity to undertake instructional improvement, these professional communities offered an inevitably partial combination of supports for teacher professional development. In particular, CFGs exerted minimal influence on teachers’ pedagogical content knowledge. CFGs would benefit from regular and systematic metacognitive and process-oriented reflections to identify how their collaborative practices might optimally advance their “bottom line” goal of improving teacher practice to increase student achievement. Additionally, high schools might pursue multiple and complementary CFG-like professional development opportunities in subject matter departments and interdisciplinary grade-level academy teams. Mid-afternoon sunlight pours into Principal Alec Gordon's living room on this early release day.1 Lounging on chairs and the carpeted floor, 11 members of Revere High School's staff—among them teachers, the principal, an instructional aide, and a counselor—are in the midst of a structured conversation about a collection of student pinhole photographs brought by Lars, an art teacher. As the group talks, some members hold and peruse the black and white matted images. One member muses aloud, “Not that Lars can answer this now, but I wonder what was the purpose of this assignment? Will doing pinhole photographs make students better photographers or is this just a fun exercise?” As required by the protocol structure, Lars sits silently listening to his colleagues’ attempt to make sense of his students’ products, as well as the instructional context that generated them. Prompted by a timekeeper, the facilitator eventually shifts the conversation. “Oh, it's time? It's time. OK, next in this protocol, we reflect on the process as a group. Share what you learned about the student, about your colleagues, about yourself. Use questions from the previous page.” As the group concludes this conversation, their 3-hour monthly meeting comes to a close. They carry cups and plates to the kitchen and gather up the papers that have accumulated in their laps and on the coffee table. Several photocopies of student essays on violence prevention, as well as copies of a Michelle Fine article, get stowed away into briefcases and knapsacks. Lars collects his students’ work, putting pinhole cameras in a bag and rolling up a poster-sized enlargement of a playground shot. Some members assemble on the deck over the water chatting; their laughter floats into the living room. Others congregate by the fireplace to share lingering ideas with Shelby, the health teacher who brought the violence prevention essays. In the dining room, a veteran math teacher approaches a first-year chemistry teacher and asks how his year is going. Meanwhile, some members scurry off, thanking their host and bidding farewell to the group.