This article develops a comprehensive understanding of how policy-makers and the private sector are conceptualizing their respective roles in national cyber security, where there may be disparity in these conceptions and what implications this may have for national and international cyber security. To this end, it begins with some necessary background to the establishment of the public– private partnership in national cyber-security strategies. It then analyses the conceptions of security that are evident in these policy documents. Unpacking the assumptions about security that drive these policies is essential to developing an understanding of the goals, objectives and embedded interests that shape the partnership. The article then moves on to analyse the public–private partnership from the perspectives of both partners. It finds that there is a fundamental disjuncture between the expectations of the two ‘partners’ in terms of roles, responsibility and authority. Disjuncture in such relationships is certainly not unique to this context, but the particular significance here arises from the fact that what is at stake is not (for example) a civil engineering project but a national security concern. The conclusion is not that no kind of public–private partnership can be central to national security in the US and UK, but rather that the partnership referred to in the policy documents is deeply flawed and that, unless the problems identified here are acknowledged and addressed, it is unlikely that this arrangement will prove a durable or effective means of promoting national cyber security.