The crusade is a feature of medieval Christian civilisation with a decidedly contemporary resonance. It summons up images and associations that have been routinely misunderstood and misappropriated, never more so than in the wake of 9/11. Many aspects of the medieval world have enjoyed, or endured, some sort of popular currency since the emergence of the Gothic novel and the Romantics’ love affair with chivalry, but few have been as badly misrepresented as the crusades. No other event or process in medieval history, moreover, has prompted modern Christian apology, both papal and evangelical. But it is not just their lurid modern reputation which makes the crusades an important area of study, for they also impinged significantly on the medieval church and medieval culture. It is necessary, for example, to know something of the crusade movement in order to understand the functioning of papal authority between the eleventh and sixteenth centuries, changes in the ideological self-fashioning of royal governments, the ups and downs in the delicate relationship between ecclesiastical and secular authorities, the spread and influence of the international religious orders, the morphology of aristocratic culture, and expressions of popular religiosity. Itself a relative novelty, crusading nonetheless insinuated itself into the most fundamental and traditional of social relations. For example, in 1201 Pope Innocent III (1198–1216) issued a decretal stating that a crusader’s vow could not be negated by a prior marriage vow.