Flood maps play an increasingly prominent role in government strategies for flood-risk management. Maps are instruments not just for defining and communicating flood risks, but also for regulating them and for rationalizing the inevitable limits and failures of those controls. Drawing on policy document analysis, official statistics, and 66 key-informant interviews, this paper explores the institutional conflicts over the use of the Environment Agency (EA) Flood Map to support decision making by English local planning authorities (LPAs), whose local political mandate, statutory obligations, and professionalized planning culture put them at odds with the narrower bureaucratic imperative of the Agency to restrict developments at risk of flooding. The paper shows how the Flood Map was designed to standardize and script the planning process and ensure that LPA decisions were aligned with EA views about avoiding development in zones at risk of flooding without actually banning such development outright. But technologies are also shaped by their users, and so the paper documents how planners accommodated and resisted this technology of indirect rule. Their concerns about sterilizing areas depicted as being at risk of flooding and about the difficulties of actually using the Flood Map for speedy and defensible development-control decisions were crucial in its eventual replacement by a new decision-support technology, Strategic Flood Risk Assessments, which then led to the descripting of the Flood Map to influence a new set of users: the public. The paper closes with some wider reflections on the significance of the case for risk-based governance.