The development of protective measures and methods of combat in armed conflict has made it increasingly difficult to render the opponent's combatants hors de combat (i.e. not able to take part in the hostilities). This has raised questions about the effectiveness of small-calibre anti-personnel ammunition. This report describes the legal framework for using small-calibre ammunition against combatants in light of this development. The aim is to create a better understanding of the conditions under international humanitarian law for using this type of ammunition in armed conflict. The report concludes that the treaty-based prohibition of explosive projectiles weighing less than 400 grams has changed since its creation, and today corresponds to the customary law prohibition of anti-personnel use of bullets which explode within the human body. The wording of the prohibition means that it does not cover anti-personnel use of bullets that explode outside the human body. The report further notes that the similarly worded treaty-based and customary law prohibitions on the use of bullets that easily expand or flatten in the human body do not cover projectiles that have other effects in the human body, such as tumbling or fragmenting. These two prohibitions are supplemented by the more general prohibition against employing weapons, projectiles and material of a nature to cause superfluous injury or unnecessary suffering, binding both as treaty law and customary international law. What constitutes superfluous injury or unnecessary suffering must be assessed in light of the objective of armed conflict to render the opponent's combatants hors de combat. Only such injury or suffering that is superfluous or unnecessary in relation to this objective is prohibited under this rule. The rules addressed in this report should be understood in light of "the principles of humanity and the dictates of public conscience" as reflected in the so-called Martens Clause. The application or this clause means that there is an ultimate protection to ensure that the requirements of humanity are not exceeded when the rules are interpreted and applied.