This chapter unpacks the heterogeneous assemblage of modes of knowledge constituted around the question of unknowability about the ‘next terrorist attack’. How does one govern what one does not know? What are the unknowns which must be made actionable through the creation and deployment of knowledge and what are the conditions of validity for truthful statements about the unknown? How are unpredictable and unexpected events made knowable? These questions are not new and by no means exclusive to the problematization of future catastrophic events. Yet, as the previous chapter has shown, worst case scenarios of catastrophic events have been taken to have caused a series of mutations in governmental and security knowledge. If labels such as speculation, premediation, imagination or enactment capture important elements of what is at stake in the governance of the ‘next terrorist attack’, the central claim of this chapter is that the term ‘conjecture’ allows us to understand the particularity of a style of reasoning which does not function through analogy with financial practices as implied by speculation, in denial of evidence as premediation would hold, or at a distance from scientific knowledge, as uses of imagination and fantasy in this context intimate.