In this pioneering work, Susanna Throop explores the role that ideas of vengeance played in the justification and conduct of crusading warfare. As she points out, this is not the first study on this topic. Several authors have touched upon this area and indeed in 2006 she published an article on this theme in the journal Crusades, which seems to have been a pilot study for this present work. This however is the first full-length treatment. Structurally, the first chapter looks at what contemporaries understood by the term ‘vengeance’, identifying the moral framework which underpinned this concept and showing how contemporaries connected it to ideas such as ‘justice’ and ‘mercy’. The following three chapters explore the changing usage and meaning of this term in three chronological phases, 1095–1137, 1138–97 and 1198–1216 (the launch of the First Crusade through to the end of Pope Innocent's pontificate). Her fundamental contention is that the idea of vengeance became increasingly predominant in crusading sources during the course of the twelfth century. This is an important finding, which challenges the existing orthodoxy that ideas of vengeance died out following the capture of Jerusalem in 1099. Several related areas are also covered, including a running discussion on the relationship between evolving ideas of vengeance and the crusaders' changing perceptions of their various enemies: Muslims, Jews, heretics and in some cases other Christians. She also explores the social context of this idea and the Church's perception of its authority to launch acts of vengeance. The final chapter changes focus slightly away from ‘vengeance’ to look at the related concept of ‘zeal’. It demonstrates that this term was likewise employed with increasing frequency during the twelfth century and that it encapsulated the motives, including vengeance, that were deemed fitting by contemporaries for the right-thinking crusader. Following the conclusion, there are appendices on the historiography of this topic and its sources. This work ‘lifts the lid’ on a key idea that was clearly central to the medieval experience, whilst demonstrating that it had connotations that we might not recognise in the modern day. Furthermore, by synthesising so much data and historiography over so long a time period, Throop has been able to contribute to some of the bigger questions concerning Christendom's culture and values during the twelfth century. She shows how concepts of vengeance played a role in the construction of a united and uniform Christendom whilst outlining the effect that this had on the identity of the crusades and its protagonists. Overall, this is a superb study that leaves a reader in no doubt that a significant advance has been made both in our knowledge of the ‘thought-world’ of the Middle Ages and our understanding of the human condition.