An international public debate over the law and ethics of autonomous weapon systems (AWS) has been underway since 2012, with those urging legal regulation of AWS under existing principles and requirements of the international law of armed conflict in argument with opponents who favour, instead, a preemptive international treaty ban on all such weapons. This chapter provides an introduction to this international debate, offering the main arguments on each side. These include disputes over defining an AWS, the morality and law of automated targeting and target selection by machine, and the interaction of humans and machines in the context of lethal weapons of war. Although the chapter concludes that a categorical ban on AWS is unjustified morally and legally—favouring the law of armed conflict’s existing case-by-case legal evaluation—it offers an exposition of arguments on each side of the AWS issue.