Through textual analysis of specific medieval vocabulary it has been possible to clarify the course of the concept of vengeance in general as well as the more specific idea of crusading as an act of vengeance. The concept of vengeance was intimately connected with the ideas of justice and punishment. It was perceived as an expression of power, embedded in a series of commonly understood emotional responses, and also as a value system compatible with Christianity. There was furthermore a strong link between religious zeal, righteous anger, and the vocabulary of vengeance. The idea of crusading as an act of vengeance largely originated in the aftermath of the First Crusade, as contemporaries struggled to assign interpretation and meaning to its success. Three themes in early twelfth-century sources promoted the idea of crusading as vengeance: divine vengeance on the unfaithful, a connection between crusading and anti-Jewish sentiment, and the social obligation to provide vengeance for kith and kin indicated by the key vocabulary of auxilium and caritas. The idea of crusading as an act of vengeance expanded noticeably through the later twelfth century. This corresponded substantially with increasing papal power, theories of material coercion, and a broad definition of the injuries committed by Muslims. The social obligation to provide vengeance was still expressed in familial terms but also was linked increasingly with lordship relations. The texts strongly downplayed the distinction between Jews and Muslims in a number of ways centring around the crucifixion of Christ, and in so doing contributed to the ideology of crusading as vengeance. In sources from the early thirteenth century, particularly papal correspondence, the idea of crusading as an act of vengeance was applied to a variety of crusading expeditions. Analysis of the idea demonstrates a strong emphasis on Christian unity and also the continued contribution of notions of social obligation. The sources continued to blur the distinctions between Jews, Muslims and heretics, again using as a binding event the crucifixion of Christ. By the early thirteenth century, the vocabulary of vengeance was an established part of crusading rhetoric.