This Article introduces a new approach to environmental law, which I refer to as “boundary work.” Legal scholars organize environmental law around a series of boundary disputes. These include: federalism, beginning with the matching principle and the appropriate scale of response to pollution; statutory interpretation, including federal power to regulate “waters of the United States” and air emissions beyond a facility; and the balance of equity and efficiency, often through critiques of risk borne by “fenceline” or environmental justice communities. These areas of inquiry focus on decisions that agencies make as they struggle with governable commons, protectable waters and habitats, controllable emissions, and manageable risks. Yet they are unable to resolve enduring puzzles in environmental protection. Among them: the failure of Title VI of the Civil Rights Act of 1964 to improve environmental quality in low-income and minority communities forty years after the Environmental Protection Agency adopted implementing regulations; the inability of state and federal agencies to determine air quality near hydraulic fracturing (“fracking”) and other unconventional oil and gas sites; and the limited ability of agencies to police entire categories of risk, such as the impacts of radioactive waste disposed in municipal landfills. These and other puzzles persist because the literature lacks a framework for the study of environmental law as it is performed—not at the level of agencies or market actors, but at a wholly distinct unit of analysis. “Boundary work” provides such a framework. Its unit of analysis is the community of practice, through which boundaries of protectable places and publics are regularly redrawn. I introduce the concept of boundary work and apply it to longstanding challenges in the areas of environmental justice, oil and gas development and production, and waste disposal. Boundary work, and the practice-oriented perspective that underlies it, points to institutional arrangements that construct, shift, and move boundaries so that they are beyond the reach of existing legal authority, irrespective of rulemaking or the settled scope of regulatory power. Boundary work’s attention to institutional change sheds light on central issues in administrative law, including the persistence of regulatory commons. It also suggests a workable application of the precautionary principle in domestic environmental law.