I will discuss a general strategy to derive Hartree's theory for the ground state of a generic interacting many-bosons system with mean-field scaling. The validity of the mean-field approximation is interpreted as a consequence of the structure of the set of bosonic density matrices with large number of particles, in particular of the so-called quantum de Finetti theorem. The approach is general and applies for example to famous examples such as the trapped Bose gas, bosonic atoms and boson stars.