The Longitudinal Spin Structure of the Nucleon

Summary. | We review flrst the parton model formalism for polarized deep inelastic lepton-hadron scattering. Topics discussed include the \spin crisis in the parton model", the role of the axial anomaly, our knowledge of the polarized gluon number density and attempts to measure it. Secondly, going beyond the simple parton model, we discuss the evolution of parton densities, the generalization of the parton model in QCD, perturbative QCD corrections and scheme dependence. Finally we comment on our knowledge of the polarized strange quark density and attempts to learn about it from semi-inclusive deep inelastic scattering. 1. { Deep inelastic scattering Deep inelastic lepton-hadron scattering (DIS) has played a seminal role in the devel- opment of our present understanding of the sub-structure of elementary particles. The discovery of Bjorken scaling in the late nineteen-sixties provided the critical impetus for the idea that elementary particles contain almost pointlike constituents and for the sub- sequent invention of the Parton Model. DIS continued to play an essential role in the long period of consolidation that followed, in the gradual linking of partons and quarks, in the discovery of the existence of missing constituents, later identifled as gluons, and in c ∞ Societµa Italiana di Fisica 1