This thesis examines the key moral principles that should govern decision-making by police and military when using lethal force. To this end, it provides an ethical analysis of the following question: Under what circumstances, if any, is it morally justified for the agents of state-sanctioned security institutions to use lethal force, in particular the police and the military? Recent literature in this area suggests that modern conflicts involve new and unique features that render conventional ways of thinking about the ethics (...) of armed conflict, and the use of lethal force, as inadequate or redundant. In particular, there is an increased concern with the moral difficulties created by “non-standard” cases. This is where the police or military are obliged to operate outside their conventional contexts. In such non-standard cases, on what moral basis can (or should) state actors – especially the police and military – use lethal force? One approach argues that there is nothing morally exceptional about the use of lethal force by police or military. This says the only available moral justification for using lethal force is killing in self-defence or defence of others. In contrast, I use an institutional approach to develop a moral framework for the state’s morally exceptional use of lethal force. The institutional approach is concerned with the ends (telos) or purpose of a social institution. It says that the moral purpose of a social institution alters the moral responsibilities of its agents; what is referred to as role morality. My analysis demonstrates that there is an important moral distinction between justified killing in self-defence and state-sanctioned uses of lethal force. My claim is that police and military uses of lethal force are not morally justified in the same way as the average person’s use of lethal force (i.e. self-defence or defence of others). Instead, I argue that the state-sanctioned institutional role of police and military give these state actors special moral duties, and therefore exceptional moral justification, for using lethal force. I argue that the institutional end of the police is the preservation of public safety. This includes using lethal force where it is necessary to protect life and prevent serious injury to jurisdictional inhabitants. In contrast, a morally responsible state uses military force to defend the “common good.” That is, when it is necessary to defend the peaceful functioning of a state from armed threats or other forms of political violence. I then conclude that non-standard cases require the addition of jus ad vim (or the just use of military force short-of-war) as a hybrid element to the moral framework for the state-sanctioned use of lethal force. This provides a better way of applying ethics to the use of military force when defending the common good against serious threats in non-standard cases. This is because jus ad vim complements the conventional military paradigm by permitting the use of military capabilities to defend the common good. But, at the same time, it inhibits the move towards the more destructive levels of violence characteristic of conventional warfighting. (shrink)