Our understanding of the physical properties of quantum many-particle systems relies normally on the identification of elementary excitations that can be considered as weakly interacting building blocks of the whole system. In systems characterized by the presence of strong interactions between the elementary components, like high-temperature superconductors and related materials or like ultra-cold atoms in optical lattices, well known theoretical techniques like mean-field or perturbation theories fail. On the contrary, quantum Monte Carlo simulations allow to handle such systems in a "numerically exact" way.