Abstract: I will present recent efforts to understand early cluster formation phases in the distant Universe, when the first giant dark matter halos were growing, and baryons falling into their deep potential wells took part to prodigiously vigorous activity of galaxy and black hole assembly. These phases are expected to be crucial to understand the processes leading to the formation of dominant elliptical population and well relaxed hot gas atmospheres, as observed in local massive galaxy clusters. New observations with ALMA, NOEMA, Herschel, HST and Keck of two dense structures at z=2 and 2.5 provide new insights into these problematics. Euclid, Athena, ALMA, SKA, JWST, Keck CWI (a bluer MUSE!) and the ELTs hold the promise to revolutionize this field in the coming decades.