The monograph is dedicated to one of the most important eras in the history of the Roman Empire and the entire ancient civilization. The profound transformations carried out by the outstanding reformer emperor led to the beginning of a new period in Roman history. The Principate, a political system that emerged during the reign of Augustus, finally transformed into the Dominat during the reign of Diocletian. “The monarchy in republican garb” becomes an unlimited monarchy. Based on a wide range of sources and research, the author analyzes the evolution of imperial power, its new system, public administration, and the economic policy of Diocletian. Particular attention is paid to the religious policy of the founder of the Dominat, the last great persecution of Christians in the Roman Empire. The book is intended for a wide range of readers.