The Proto2Clusters project is investigating the formation of the most massive systems in the Universe, exploring a critical epoch when large-scale protocluster structures collapse to form the virialized galaxy clusters observed at later cosmic times. With their extreme environments, galaxy clusters offer a prime opportunity to investigate outstanding questions in extragalactic astrophysics: What is the role of environment in driving the evolution of galaxies? What is the role of galaxy star formation and nuclear activity in shaping the properties of the intracluster medium? What is the connection between AGN on sub-parsec scales, galaxy populations, and the surrounding large scale structure on scales of hundreds or thousands of kiloparsecs? While a "concordance picture" of the cluster evolution out to z~1 is now broadly established, the earlier phases of their formation bridging protocluster environments at z>2 and the virialized clusters at z~1 are far more complex, uncertain and still controversial. Our team has designed and analysed multi-wavelength observations and state-of-the-art theoretical models of structure and galaxy formation and evolution for a proper interpretation of observational results, in a broader cosmological context, of the transition phase between protoclusters and their massive and virialized descendants observed about 1 Gyr later.