The Introduction explains the relevance of a theoretical inquiry into the purpose and function of belligerent reprisals. It highlights several examples in recent practice where the vocabulary of belligerent reprisals has been harnessed by parties to an armed conflict, pointing to the continued relevance of the institution in contemporary warfare. At the same time, it outlines persisting difficulties in the terminology, regulation and governance of reprisals, and shows that they all derive from the failure by international legal theory to give a proper legal vest to the purpose and function of the mechanism. It points to fundamental fallacies both in how the question has been approached, and in how it has been answered. It proposes an alternative to existing accounts and outlines how it will be investigated in the book.