Civilians are playing an increasingly decisive role in determining the outcome of wars. This has further blurred the fundamental civilian/combatant distinction that is the cornerstone of modern humanitarian law. The lawfulness of an attack on civilians depends on their own conduct in hostilities, and hinges on the principle of "direct participation in hostilities". The definition and scope of this concept are the subjects of debate, to which the Interpretative Guidance of the ICRC, published in this edition of the Review, aims to bring further clarification.