The fundamental aim of this thesis is to test three things. First, whether there can be a ‘rule of law’ in the international humanitarian law (IHL) and international human rights law (IHRL) rules that regulate the use of lethal force by state militaries (Rules of Targeting). Second, whether there should be such a rule of law, and third, whether there is one. These questions matter because they allow us to consider what is important about the rule of law and whether and, if so, how the rule of law can be applied within the context of an armed conflict. I have chosen to focus on targeting decisions by state militaries, in the context of international armed conflicts (IAC) – conflicts between two or more states. This is because it forms the paradigm case for which the law in question is designed. In this thesis I set out why there can be a rule of law regulating the use of lethal force in IACs, why – to a limited but non-trivial extent – there currently is such a rule of law, and why it is a worthwhile endeavour to attempt to apply the rule of law to such exercises of power.