  We review recent developments in the theory and phenomenology of polarized
structure functions. We summarize recent experimental data on the proton and
deuteron structure function $g_1$, and their impact on the understanding of
polarized sum rules. Specifically, we discuss how accurate measurements of the
singlet and nonsinglet first moment of $g_1$ test perturbative and
nonperturbative QCD, and critically examine the way these measurements are
arrived at. We then discuss how the extraction of structure functions from the
data can be improved by means of a theoretical analysis of their $x$ and $Q^2$
dependence, and how, conversely, experimental information on this dependence
can be used to pin down the polarized parton content of the nucleon.
