Rewilding is a conservation approach that is gaining increasing attention from academics, public opinion-makers, and policy makers. But what is rewilding? A large number of academic definitions coexist along with many different on-the-ground practices, and a lack of clarity at the policy level. Here, we trace the transformations of rewilding between practice and policy, following an anthropology of policy approach. We also enroll the "policy assemblage" concept to ask how rewilding is mobilized and how rewilding actors change their relations within assemblages across these practice-policy transformations. We look at how rewilding is made into a reality at particular sites, as projects. We compare this to how rewilding is understood, positioned, used and produced at the policy level. We use a comparative approach, looking at projects in the UK, the Netherlands and Denmark, as well as policy frames and interactions in each of these countries and at the EU level. In order to secure future funding flows and institutional security, projects eventually (or immediately, in Denmark) present themselves as successful and normative within the existing regulations defining success or acceptability. At the same time, the practice of rewilding and the policy of rewilding define failure in different ways. At the project level, flexible long-term goals and adaptive learning allow failures to be avoided by definition. For policy makers, the fear of failure leads to avoidance of projects that cannot be defined as successful. We note that if high-level policy eventually defines normative success in rewilding it will probably do so in terms of its technical practices of reintroduction and passive management. It will be much more challenging to enshrine in policy the "meta-practices" of flexibility, long-term thinking and adaptive learning, which ensure rewilding success in practice. A broad acceptance of rewilding, as it is mobilized between projects and policies, may lead to radical changes in how rewilding is enacted.