This special issue, which includes a translation of part of the conclusion of Serres’ thesis on the emergence of ARPANET (a work which is relatively unknown in the English-speaking community), two original articles and two interviews, is linked to the celebration of the 50th anniversary of the ARPANET and the desire to reconsider, 50 years on, the birth of this heterogeneous network that has left such a deep mark on the history of data networks. If we consider ARPANET as a first step towards the Internet, how did it determine – and how does it continue to determine – our current understanding of the Internet? What is left to explore and discover in ARPANET’s history? And if ARPANET is dead today, are its legacy and spirit still alive, and if so, in what way? While the few texts collected are of course not intended to provide all the answers, we hope that they will at least give food for thought.