Like many other parts of the public sector, the French national police has been subjected to the managerial ambitions of state reformers. These have included the 2004 reform of the police corps, which sought to make of the national police the “avant-garde of modernization.” This article examines this police reform with a focus on uses of the reform by some police unions depending on their position within the institution and their resources. It investigates the concrete forms of the diffusion of a managerial model in a public sector through the study of the role played by unions. Indeed the spread of a “dominant ideology” is far from a natural process: it requires indispensable allies and actors to shape and diffuse managerial tools. These may include some police unions (of officers and commissioners) that have an interest in promoting the reforms.