The present situation in deep inelastic lepton-nucleon scattering is reviewed from an experimenter's point of view . After describing. in some detail. the quark-parton model (QPM) predictions the basic ideas of Quantum Chromodynamics (QCD) are illustrated . The perturbative QCD predictions for deep inelastic scattering are summarised discussing also their theoretical uncertainties of today .Past and present experiments on deep inelastic electron and muon scattering are compared regarding their ability to test these predictions . It is shown that all data is in very good agreement with the QPM and fully consistent with perturbative QCD . New results on lepton scattering off nuclei are discussed and confronted to recently invented phenomenological models .