Prevention is an attractive idea to policymakers in theory, particularly in health where the burden of spending and care is increasingly taken up by complex and chronic conditions associated with lifestyle choices. However, prevention in general, and preventive health in particular, has proven hard to implement in practice. In this paper, we look to one tangible legacy of the recent rise of the prevention agenda: agencies with responsibility for preventive health policy. We ask how this form of institutionalizing preventive health happens in practice, and what consequences it has for the advancement of the prevention agenda. We draw on qualitative data to compare the trajectories of newly formed agencies in Australia, New Zealand and England. We find that building and maintaining legitimacy for such agencies may come at the expense of quick progress or radical action in service of the prevention agenda.