This little volume is a study of the relations of the Popes and monarchs (particularly the Holy Roman Emperors) of 10th and 11th century Western Europe. There was no separation of Church and State. The kings had quasi-sacerdotal functions amd the Popes were important in the politics of the time. In the 9th and 10th centuries Charlemagne's Carolingian empire had fragmented due to being divided among his heirs, and Western Europe was also under attack by Vikings from the north, Magyars (Hungarians) from the east and Muslims from the south. The Popes in Rome were controlled by the clan factions of the Roman nobility. In the later 10th century German Kings (who were also the Holy Roman Emperors) gained control of the papal elections and began the reform of the Papacy. But then a struggle began between the Popes and Emperors as to the nature of their relationship, what was the right ordering of church/state relations. The papacy of Gregory VII was the turning point of this era when advanced claims for the Papacy were claimed and set forth. The long term consequences were enormous. Up to then, kings were sacral figures, partly an inheritance from Germanic paganism, but also due to their anointing by the bishops in the coronation ceremonies (based on Old Testament precedents). The claim to papal primacy over kings was a long term blow to sacral kingship, although the Divine Right of Kings survived for centuries and was a long time disappearing. What I have just written is an oversimplified outline of some of the main points and there is much more detail discussed with a lot of nuance, which is absolutely necessary for understanding the events of this important epoch of Western history.