[en] Deep inelastic scattering of leptons off hadrons has proven to be an excellent tool to probe the elementary structure of hadrons. It has shown the 'existence' of quarks in the nucleon. It has also provided one of the clearest test of the fundamental theory of the strong interactions, quantum chromodynamics (QCD). The above main aspects of deep inelastic scattering will be discussed in this chapter. In chapter two a general introduction to one specific model, the spectator model will be given. In a simple picture of the nucleon the spectator is a diquark system. Using field theoretical methods one is able to treat all the kinematics in a correct way and assure the validity of QCD-based sum rules. In chapter 3 the effects of interactions between quarks and gluons and the subsequent Q*2* evolution of structure functions are treated, as well as some of the problems arising at small x. This will be applied to the diquark spectator model in chapter 4, leading to various results that can be compared to the experiments. Finally the application of the same formalism to nuclear structure functions is treated in chapter 5 in connection with quark exchange effects in nuclei. 68 refs.; 31 figs.; 6 tabs