Autonomous weapon systems (AWS), employed in swarms, promise enhanced military effectiveness. However, with that efficiency comes complexity in understanding how those swarms operate and how their use might comply with international law. States have agreed that the use of AWS must conform with international law and, more relevantly, the laws of armed conflict. Prior to the use of weapons, means and methods of war, States must ensure whether as a legal obligation under Additional Protocol I or as a matter of practicality that new weapons, means and methods of war are capable of complying with their international legal obligations. This chapter explains how the use of simulation testing and evaluation methods can be used to support the conduct of these legal reviews. It identifies which agents of an AWS, and each agent’s constituent parts, must be the subject of review and concludes that to properly undertake an assessment of the lawfulness of AWS swarms, reviewers must take a risk-based approach. This risk-based approach can be informed by simulation testing techniques, such as Monte Carlo methods, and will aid in assisting reviewers to understand what limits must be placed on AWS swarms to make them lawful, such as limitations on swarm size or mandating of detection, classification, recognition and identification confidence level to undertake particular combat actions.