Given the swift technologic development, it may be expected that the availability of the first truly autonomous weapons systems is fast approaching. Once they are deployed, these weapons will use artificial intelligence to select and attack targets without further human intervention. Autonomous weapons systems raise the question of whether they could comply with international humanitarian law. The principle of proportionality is sometimes cited as an important obstacle to the use of autonomous weapons systems in accordance with the law. This article assesses the question whether the rule on proportionality in attacks would preclude the legal use of autonomous weapons. It analyses aspects of the proportionality rule that would militate against the use of autonomous weapons systems and aspects that would appear to benefit the protection of the civilian population if such weapons systems were used. The article concludes that autonomous weapons are unable to make proportionality assessments on an operational or strategic level on their own, and that humans should not be expected to be completely absent from the battlefield in the near future.