Polarized structure functions: theory and phenomenology

We review the theory of polarized structure functions measured in deep-inelastic lepton-nucleon scattering, focusing on the most recent developments. We concentrate on the structure function g_1, emphasizing the phenomenological problems related to the extraction of the proton and neutron g_1(x,Q^2) from the data, and to the determination of its moments, especially the first moment. In particular, we discuss the theoretical and experimental uncertainties due to the Q^2 dependence of the data, small and large x extrapolations, QCD loop corrections, higher twist corrections, and, in connection to neutron experiments, nuclear effects. We also discuss the current status of the sum rules satisfied by the first moment of g_1 and their theoretical interpretation.