Trends in Polarized Lepton Nucleon Scattering

The thoery of scattering between polarized leptons and polarized nucleons is reviewed in a comprehensive and pedagogical way. The polarized structure functions are introduced and their interpretation in the framework of the QCD parton model is discussed, including the gluon contribution. Sum rules ( = relations between moments of the structure functions) are derived and critically considered. They are then contrasted to recent experimental results.