This article takes the history of teacher research in one large urban school district over a period of 20 years as a telling case of the intensely local character of this work. Beginning with an overview of the variations and different conceptions of teacher research in the USA, we argue that teacher research is continually being invented and reinvented by participants in the movement and is strongly informed by local conditions, agendas and epistemologies. Using a multi-voiced text juxtaposed with published accounts, focused interviews and institutional histories, the narrative highlights work in three time periods, showing how teacher research has been within, against, and beyond the system, explicitly embedded in larger and local social and political contexts. The defining characteristic of teacher research in Philadelphia has been its primary commitment to improving the life chances of urban students and schools in a complex, embattled, and continually restructuring system. Although teacher researchers have privileged its immediate relevance, value, and impact in the situations where they work, their efforts may have common purchase with distal groups of teachers in the USA and beyond who also seek to use critical inquiry to initiate and sustain equitable change. (Contains 10 notes.)