Who are the people of Europe? This question is facing statisticians as they grapple with standardising national census methods so that their numbers can be assembled into a European population. Yet, by so doing—intentionally or otherwise—they also contribute to the making of a European people. This, at least, is the central thesis of ARITHMUS. While typically framed as a methodological or statistical problem, the project approaches this as a practical and political problem of assembling multiple national populations into a European population and people.Why is this both an urgent political and practical problem? Politically, Europe is said to be unable to address itself to a constituted polity and people, which is crucial to European integration. Practically, its efforts to constitute a European population are also being challenged by digital technologies, which are being used to diversify census methods and bringing into question the comparability of national population data. Consequently, over the next several years Eurostat and national statistical institutes are negotiating regulations for the 2020 census round towards ensuring 'Europe-wide comparability.'ARITHMUS will follow this process and investigate the practices of statisticians as they juggle scientific independence, national autonomy and EU comparability to innovate census methods. It will then connect this practical work to political questions of the making and governing of a European people and polity. It will do so by going beyond state-of-the art scholarship on methods, politics and science and technology studies. Five case studies involving discourse analysis and ethnographic methods will investigate the situated practices of EU and national statisticians as they remake census methods, arguably the most fundamental changes since modern censuses were launched over two centuries ago. At the same time it will attend to how these practices affect the constitution of who are the people of Europe.