As increasingly automated—and in some cases fully autonomous—weapon systems enter the battlefield or become possible, it is important that international norms to regulate them head down a path that is coherent and practical. Contrary to the claims of some advocates, autonomous weapon systems are not inherently illegal or unethical. The technologies involved potentially hold promise for making armed conflict more discriminating and causing less harm on the battlefield. They do pose great challenges, however, with regard to law of armed conflict rules regulating the use of weapons. To adapt existing law to meet those challenges, we propose a three-tiered approach to emerging autonomous weapon technologies.