CHEP 2021: Preface to the Proceedings Catherine Biscarat, Simone Campana, Benedikt Hegner, Stefan Roiser, Chiara I. Rovelli and Graeme A. Stewart EPJ Web Conf., 251 (2021) 00001 Published online: 23 August 2021 DOI: 10.1051/epjconf/202125100001