In March 2019, the Onsekiz Mart University of Çanakkale and the Westfälische Wilhems-Universität Münster hosted a conference entitled „Imperial Residence and Site of Councils. The Metropolitan Region of Nicaea / Nicomedia from the Severan to Constantine Period“. The western Bithynian city pair immediately evokes prominent associations in Classical Studies. Juxtapositions of ‘political vs. religious’ or ‘pagan vs. Christian centre’ are, however, too simplistic. Coexistence and rivalry between the two neighbouring cities can be traced back to their Hellenistic origins and have shaped the history of this key region between Europe and Asia for centuries up to the present day. In this volume a closer look is taken at the two cities, each in its own right, but also at their relationship to one another drawing a new picture of their urban histories. Well-known monuments and sensational new finds from both cities are discussed in the context of cultural milieu and urban life. The contributions reveal an updated overall view of their historical spaces with entailing political, social, administrative, economic and religious structures and developments. Through an interdisciplinary approach synthesizing the findings of recent archaeological, epigraphic, numismatic, historical, and geographical research, the characteristics of the neighbouring cities of Nicaea and Nicomedia as well as their interactions with each other during the transitional period from the Principate to Late Antiquity are elucidated.