In this article, it is argued that the constitutive principles of Just War Theory and the jus ad bellum/jus in bello duality do not transfer all that well to national security intelligence activity. Accordingly, while Just Intelligence Theory–comprising jus ad intelligentiam and jus in intelligentia–provides a useful starting point in the construction of a normative framework for national security intelligence activities, ultimately it is found to be wanting in a number of important respects. For instance, while war ought to be a last resort, intelligence collection and analysis ought to be a first resort. In addition, analyses are offered of the key principles of discrimination, necessity and proportionality, and it is shown in general terms how they apply, or ought to apply, to national security intelligence activity. Importantly, the principle of necessity has been given a novel analysis according to which it is in reality a set of different principles, depending on the institutional setting in which it is being used. Moreover, the analysis reveals that as typically used it consists (in part) of a means/end principle of rationality and one or other versions of a principle of harm minimisation. Finally, it is argued that there is a normative principle governing espionage, in particular, that is not a constitutive principle of Just War Theory; this is a principle of reciprocity.