The focus of this thesis is the normative base of moral liability to defensive harm. Many argue that liability is what makes it morally permissible to seriously injure or kill in self-defense or in the defense of others. Authors such as Jonathan Quong and Jeff McMahan argue that liability not only has important implications for the individual morality of self-defense, but that it plays a major role in the principles of just war conduct. How you determine when someone is liable will have a significant impact on when someone can be harmed. In this paper, I focus on the question of what a person must do to be morally liable to defensive harm. More specifically, I take a close look at Jeff McMahan’s moral responsibility account of liability and argue that it is unsatisfying as an explanation of when and why a person is liable. I then argue that an evidence-based account of liability better captures our moral intuitions surrounding liability. I end by considering an argument put forward by Quong on why we should not support an evidence-based account of liability.