This thesis offers an empirical examination of the programme of state modernization developed in France, between 2014 and 2017, around the concept of the ‘Platform State’. It examines the state’s interest for the technological modernity of public administrations – often labelled digital. The state modernisers’ mandate was as follows: simplify the implementation of administrative procedures, assess and improve the quality of public services for users, invest in digital infrastructure, experiment with new modes of government, involve users and public officials. This programme did not claim to transform the state as a whole. I argue that it constituted rather a technological swelling on the administrative landscape, a fold. To account for this new world of modernisation and its political and material consequences, my work is positioned at the crossroads of science and technology studies, the sociology of activity and the sociology of the state. Based on an ethnographic study carried out in the Prime Minister’s department in charge for the coordinated activity of state reform, I analyse this modernisation in-the-making as a process, and a project. The thesis starts with review of the literature on the reform of the state and the phenomena of modernisation. It is then divided into four chapters, each analysing one specific project. All projects, instrumented by design, ergonomics, computer science, economics and sociology, engage different facets of the state’s transformation, and explore different sites of the state.