In many respects this book, first published in 1961, marked a somewhat radical departure from contemporary historical writings. It is neither a constitutional nor a political history, but a historical definition and explanation of the main features which characterised the three kinds of government which can be discerned in the Middle Ages - government by the Pope, the King, the People. The author's enviable knowledge of the sources - clerical, secular, legal, constitutional, liturgical, literary - as well as of modern literature enables him to demonstrate the principles upon which the papal government, the royal government, and the government of the people rested. He shows how the traditional theocratic forms of government came to be supplanted by forms of government based on the will of the people. Although concerned with the Middle Ages, the book also contains much that is of topical interest to the discerning student of modern institutions. Medieval history is made understandable to modern man by modern methods.