Based on state of the art Quantum Monte Carlo simulations, we investigate the metallic states of the one-dimensional t-J model and of a depleted Kondo lattice model in two dimensions. In the one-dimensional case, it is known that correlation effects invalidate the Fermi liquid picture and that the elementary excitations are spinons and holons carrying separately the charge and spin of the electron. In this dimension we will present new results on the single particle spectral function and discuss the implications of spin-charge separation on this quantity. The Kondo lattice model describes heavy fermion materials which generically have Fermi liquid ground states but with effective masses up to three orders of magnitude larger that the bare electron mass. On the basis of numerical simulations, we will show how this heavy fermion state comes about and set the emphasis on the coherence temperature.