This book aims to make accessible the sources and controversies concerning a key period in the history of the Roman Empire -- the reign of Diocletian and its immediate aftermath. Diocletian was an emperor of unusual ambition, and his reign saw considerable military success, an experiment in collegiate government, a move towards provincial capitals away from Rome, a reorganisation of the administrative machinery of empire and its finances, and a committed project to persecute the Christians. In Part I, an introduction to Diocletian and the world of the late third century is followed by six thematic chapters covering a range of aspects of government and society under this emperor, including military, economic, religious and administrative affairs. These chapters discuss the original sources, highlight their strengths and weaknesses, and consider the main scholarly approaches to them. Throughout Part I there are regular cross references to the source material which is presented in Part II -- this includes literary, archaeological, artistic, legal, and documentary evidence, as well as coins and inscriptions. All texts are in English, and there is a guide to further reading, a full bibliography, some questions for consideration, a glossary of technical terms, and a brief list of relevant online resources.