This chapter presents an overview of key empirical research examining ethical and unethical behaviour in the military. Early research examined the impact of atrocities on the psychological well-being of Vietnam War veterans. In later conflicts, researchers examined battlefield attitudes, behaviours, willingness to report fellow unit members for ethical violations, and the adequacy of training. In addition to battlefield ethics, researchers have also investigated individual, situational, and organizational factors that increase the risk of unethical behaviour (i.e., ethical risk factors). This chapter summarizes research that highlights how individual differences in moral identity and malevolent traits can impact ethical and unethical behaviour among military personnel. It also discusses how situational factors, such as sleep deprivation and anger, can increase the likelihood of military personnel engaging in unethical behaviour. Lastly, the chapter discusses how organizational factors, such as ethical climate and culture and ethical leadership, play a role in ethical and unethical behaviour.