The DANUBIUS project proposes to study the Christianisation of the Lower Danube over a very long period, between the 3rd and the 8th centuries AD. That research topic has not resulted in any complete synthesis since Jacques Zeiller’s Les origines chrétiennes dans les provinces danubiennes de l’Empire romain (Paris, 1918). This projet – which is the first stage of a long-term international and interdisciplinary research program – will thus reconsider the question in the light of the scientific advances made in the last hundred years, both in terms of history and archaeology, for a better understanding of the simultaneous evolution of its ecclesiastical organisation and Christian topography. At first, it will focus more on the last reaches of the Danube to be lost by Constantinople, which is also the part of the river which is best documented from literary sources, viz. the Lower Danube (the Late Roman provinces of Dacia Ripensis, Moesia Secunda and Scythia). To do so, it will develop a geographic information system (GIS) model of the episcopal sees and other Christian sites, structured around three axes. The first axis foresees the compilation of a database of the written and material evidence of Christianisation. The second axis is planned around the archaeological explorations of a little-known Late Roman site, viz. Zaldapa (Krushari, Bulgaria), which may be unique in being the only episcopal see of the region planted in an agglomeration that predates the Romans. The third axis, proceeding from part of the documentation collected in the other two, intends to offer a prosopography, as the first step towards the publication of the Balkan volumes of the Prosopographie chrétienne du Bas-Empire (PCBE) series. Globally, this collection of data and case studies will be the laboratory for innovative conceptual tools and a new synthesis.