As Cardinal Hugolino and as pope, Gregory was one of the dominant figures in the history of the papacy of the High Middle Ages. His pontificate was a key stage in the development of papal relations with many of the realms of Christendom, as well as in legal and administrative history, the battle against heresy (especially with the foundation of inquisitions), the crusades, and the flowering of the Mendicants. Hugolino came to prominence during the pontificate of his relative, Pope Innocent III, and played an important political role, particularly as legate on various occasions, as well as being a major promoter of the new religious orders. As pope, his battle with Emperor Frederick II is one of medieval history's most absorbing conflicts (though one which rarely receives a balanced treatment). But he also acted as peacemaker in England, as promoter of the crusades in the East and in Iberia, where he met with considerable success, as instigator of mission for the sake of conversion, as a refomer of the Curia, and as a passionate advocate of Church reform generally. His decretal collection, compiled by Ramon de Penyafort, served as the canon law of the Catholic Church from 1234 until 1917. Yet Gregory has not received much attention since an Italian biography by Salvatore Sibilia was published half a century ago and a full examination of his pontificate is now very long overdue. The current volume brings together a team of international scholars, each of them expert in dealing with a particular aspect of the pontificate, and provides what will be a volume of lasting scholarly value on a central figure of the medieval papacy.