We verify Bogoliubov's approximation for translation invariant Bose gases in the mean field regime, i.e. we prove that the ground state energy E N is given by E N = N e H +inf σ (H)+o N →∞ (1), where N is the number of particles, e H is the minimal Hartree energy and H is the Bogoliubov Hamiltonian. As an intermediate result we show the existence of approximate ground states Ψ N , i.e. states satisfying H N Ψ N = E N + o N →∞ (1), exhibiting complete Bose-Einstein condensation with respect to one of the Hartree minimizers.