Background/Context: In an era of high stakes accountability, public school districts struggle to improve teaching and learning for all students. As a result, effective professional development approaches for teachers are a high priority. Recently, teachers' learning communities (LCs) have been recommended because successful LCs foster teacher collaboration and make practice public. At a deeper level, however, this type of professional development depends on teachers taking more control over their work, releasing tacit knowledge and expertise, developing critical judgment, and taking fuller responsibility for student learning. Such a construction of teachers' roles and responsibilities sometimes collides with entrenched norms in school cultures. Purpose: This article explores four core themes, which represent endemic challenges to sustaining teachers' learning communities (LCs): 1) defining and fostering teacher agency, 2) determining purposes for teacher collaboration, 3) tracking the challenges to and impact on district culture, and 4) identifying enabling and constraining institutional and policy conditions. The author uncovers conflicts that frequently emerge when efforts at enhancing the professional autonomy, authority, and responsibility of teachers conflict with hierarchical and bureaucratic district and school cultures. Setting: The study is located in a mid-Atlantic city in the United States, struggling with economic disparities, entrenched poverty in some sectors, a shifting economic base, and rapidly changing demographics largely due to immigration. The school district faces challenges typical of other urban districts-closing the achievement gap between middle class and poor children; developing culturally responsive educational practices, providing adequate resources in uncertain economic times; and meeting intensifying federal and state accountability demands. Population: Research participants include the district superintendent, district administrators, principals, instructional coaches, and teachers. Research Design: This article, based on two and a half years of data collection (October 2001 to April 2003), draws on site-visit interviews and focus groups of key players, observations of LC participants' meetings and classrooms, e-mail correspondence with several key players, observations of LC coaches' trainings, and reviews of relevant documents. The author served as an outside researcher to track the district's implementation of the initiative. Eventually, the field-based data was compared with survey data with responses from 251 LC participants in the district. Survey questions were designed by a research team, which included three other outside researchers studying the same initiative in three districts in other states. In all, the qualitative data required visits to LC coaches' trainings and five on-site visits to the school district, each visit extending over two to four days. Conclusions/Recommendations: From the data, the author draws several conclusions with implications for the initiative's success and sustainability. First, although the initiative sought to establish learning communities to mobilize practitioner expertise and build collective responsibility--all for the sake of student learning--most participants did not claim a connection between their collaborative work and student learning. Second, while the district has made considerable headway institutionalizing structural dimensions of the initiative, efforts to enhance teacher efficacy appeared to be constrained by high-stakes accountability policies requiring compliance. Third, within the groups, more time was devoted to community-building efforts than to critical inquiry aimed at improving practice. Fourth, because the initiative's practices and principles run counter to entrenched norms of district culture, its sustainability may be in question. Fifth, paradoxically, district leadership, though seeking a promising context for change, may be unwittingly causing conditions that threaten to undermine the initiative. Finally, if an initiative like this is to endure, districts must invest greater authority and autonomy in participants as well as adequate time and support.