The papacy is as old as the Christian Church itself, and a crucial influence on the political history of medieval and modern Europe. Pope Leo III"s coronation of Charlemagne established the first real imperial power in the West since the fall of Rome and created the model for what would become the Holy Roman Empire; Pope Gregory VII"s dispute with the German Emperor over the appointment of church officials exerted a huge influence on the course of medieval German history; the preaching of Pope Urban II led to the First Crusade and centuries of war between the Christian West and Islam; in the 20th century Pope Pius XI signed the Lateran Treaty with Italy, establishing the Vatican City as a sovereign state. And even today, the Roman Catholic Church"s teaching - as expressed through papal pronouncements on matters as diverse as abortion, contraception, terrorism and the problem of global poverty - continues to impact on the lives of billions of people around the world. The 256 occupants...