With this special issue of Defence Studies, we situate defence planning as a constitutive element of defence and strategic studies. Indeed, in addition to the usual “downstream” focus on the use or non-use of force, on policy decision-making in foreign relations, military operations and global external engagement, we argue for the utility of an increased “upstream” focus on what is a major part of everyday defence and security policy practice for military, civilian administrative and political leadership: the forward-looking preparations for the armed forces and other capabilities of tomorrow. In particular, the special issue contributions explore two general dimensions of defence planning: the long-term, historical relationship between defence planning and the state including national variations in civil-military relations, and a concurrent tension between defence planning as an administrative, analytically neutral activity and the politics of its organisation and outcomes. In both of these, defence planning appears as a particular case of general planning, as a lens that enables particular foci on the external world to come about on behalf of the state while also sometimes creating institutionalised biases along the way. In this manner, paraphrasing Émile Durkheim, defence planning emerges as a “strategic fact” with dynamics of its own.