  We review the present status of polarized structure functions measured in
deep-inelastic scattering. We discuss the x and Q^2 dependence of the structure
function g_1, and how it can be used to test perturbative QCD at
next-to-leading order and beyond. We summarize the current knowledge of
polarized parton distributions, in particular the determination of the first
moment of the quark and gluon distributions and the axial charge of the
nucleon. We critically examine what future experiments could teach us on the
polarized structure of the nucleon.
