This introduction presents an overview of the key concepts discussed in this book. The book investigates the ways in which the crusades have been observed by historians from 1090. Especial emphasis is placed on the academic after-life of the crusades from the sixteenth to twenty-first centuries. The book attempts to describe the history of the different opinions, how they have been adopted and their historical contexts. Few events of European history have captured the sentiments of contemporaries and the imaginations of later observers more vividly than the series of Christian wars now known as the crusades. Traditions, ideology and institutions grew up thickly around these wars. The debate in the nineteenth and early twentieth centuries revolved around essentially materialist interpretations, the crusades as wars of conquest, motors of economic expansion and expressions of colonialism.