This chapter proposes a brief lexical history of the English term ‘responsibility’, starting from the Latin ‘respondere’, through French and Anglo-Norman, up to the emergence of the abstract noun in modern languages. Seen from a purely semantic diachronic perspective, ‘responsibility’, as a legal term, expresses the idea that one may be called upon to answer for one’s activity (or inactivity) but does not itself denote the prerequisites of liability nor hints at the source or nature of any duty to do so. It looks at what is to be done in the future, not to the imputation of a duty by reference to what has gone before. If the meaning that history brings to light is quite neutral – and pivots around the ordinary, basic meaning of the verb ‘respondere’: ‘to say something in answer to a question’ – ‘responsibility’ appears nonetheless to be most appropriate to signify the distinctive feature of contemporary international law, namely that States ‘have to answer’ for their actions.