Abstract: One of the deepest difficulties faced by belligerents and their individual members is the problem of uncertainty: how can they know whether their war, and individual acts within the war, meet the requirement of the just war? For proponents of the view that soldiers may not kill enemy soldiers unless their ends are just, the problem is particularly acute. In this paper, rather than ask what agents may justifiably do in war, and what may justifiably done to them in return, given that they do not know the facts of the matter, I instead address a question which, oddly, has been consistently overlooked in just war theory – namely whether they may justifiably improve their epistemic circumstances in the first instance by spying on one another. At first sight, this might an easy question: to the extent that we are permitted to go to war and kill in it, surely we are permitted to spy – are we not? Yes, and no. I argue that under certain conditions (and only under those conditions) belligerents are pro tanto justified, indeed are under a pro tanto obligation, to spy on their enemy. As we shall see, defending that claim requires more argumentative work than merely settling on the morality of a particular war and/or of its constituent harmful acts. As we shall also see, pondering on the morality of war-time espionage (an interesting ethical question in and of itself) enables us to explore some of the epistemic dimensions of the ethics of war.