  We present an introductory discussion of deep-inelastic lepton-proton
scattering as a means to probe the substructure of the proton. A resume of QCD
is given, emphasizing the running of the coupling constant and the DGLAP
evolution equations for the parton densities. The determination of parton
distributions is discussed and their importance for predictions of processes at
the LHC is emphasized. Going beyond the pure DGLAP regime, we briefly discuss
the behaviour of parton densities at low x, and the evidence for non-linear
absorptive contributions.
