We now know that the first massive groups and clusters were already forming in the first 2-3 billion years of the Universe’s history. Unlike their modern-day descendants, these fascinating systems contained often highly star-forming gas-rich galaxies that later on quenched (stopped forming stars) and transformed morphologically into ellipticals systems through processes that are not yet fully known. This include the recent discovery of peculiar systems with quenched disks and star-forming bulges, which could be seen as ‘anti-galaxies’ respect to the later-time typical objects that are quenched in the centres (bulges) and forming stars in the disks (like our Milky-Way Galaxy). Other exotic components that have been recently discovered are giant reservoirs of cold diffuse hydrogen, possibly connected to cold streams postulated by theory and never fully confirmed, and intracluster light, probably related to early phases of galaxy interactions and mergers that stand in contradiction with current model predictions. The Euclid satellite, with a strong participation from France and CEA/AIM, will be launched in 2023, and will provide ideal ultra-deep multi-wavelength imaging in the optical and near-IR, together with ancillary data at longer wavelengths, to identify the first generation of galaxy groups and clusters that started forming in the distant Universe and study the different exciting physical phenomena that are occurring in dense environments at early times. The PhD student will join Euclid teams and will lead research into these problematics in collaboration with the group in CEA Saclay.