Fully autonomous weapons, also known as "killer robots," would be able to select and engage targets without human intervention. Fully autonomous weapons do not exist yet, but they are being developed by several countries and precursors to fully autonomous weapons have already been deployed by high-tech militaries. Some experts predict that fully autonomous weapons could be operational in 20 to 30 years. These weapons would be incapable of meeting international humanitarian law standards, including the rules of distinction, proportionality, and military necessity. The weapons would not be constrained by the capacity for compassion, which can provide a key check on the killing of civilians. Fully autonomous weapons also raise serious questions of accountability because it is unclear who should be held responsible for any unlawful actions they commit. Human Rights Watch calls for a preemptive prohibition on fully autonomous weapons.