The crisis period of the late 3rd and early 4th centuries AD is central to the history of the Roman Empire. The emperors Diocletian and Constantine played a special role here, since Constantine's pro-Christian religious policy was in clear opposition to the policies of his predecessor. In ancient historical research, the political and personal-religious reasons that prompted the protagonists to act and the effects of their actions on state and society are extremely controversial. Against this background, the volume would like to make a contribution to concisely taking stock of various fields of research on late antiquity, critically examining their paths and wrong turns and pointing out worthwhile perspectives for future research. The twelve contributions deal with central aspects of religious history and political history, which are analyzed on the basis of literary, epigraphic and numismatic sources. The focus is on three major fields of research: Diocletian and the Tetrarchy, Constantine and his religious policy, and the consequences of the "Constantine turn" for late antiquity up to the Emperor Justinian.