DANUBIUS intends 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). The project – which is the first stage of a long-term international and interdisciplinary research programme – is thus reconsidering 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 ecclesiastical organisation and Christian topography in the Danubian world during Late Antiquity. The study is focusing on the last reaches of the Danube to have been lost by Constantinople, which is also the part of the river which is best documented from literary sources, viz. the territory covered by the Late Roman provinces of Dacia Ripensis, Moesia Secunda and Scythia, together with Cherronesus Taurica, as a bordering comparative element outside the Empire. To reach the targets, a geographic information system (GIS) model of the episcopal sees and other Christian sites, structured around three work packages (WP), is in production. The first WP foresees the compilation of a database of the written and material evidence of Christianisation. The second WP is planned around the archaeological explorations of a little-known Late Roman site, viz. Zaldapa (Krushari, Bulgaria), which is unique for the study of Christianity in the region, as it is a very large fortified city with several churches, abandoned at the beginning of the 7th century and never really reoccupied since. The third WP, proceeding from a part of the documentation collected in the other two, intends to offer a prosopography as the first step towards the publication of the volumes of the Prosopographie chrétienne du Bas-Empire (PCBE) series for Central and Eastern Europe. Globally, this collection of data and case studies is the laboratory for innovative conceptual tools and a new synthesis; these results will ultimately be offered in open access, thanks to the D(OA)NUBIUS Initiative.