  We present a Next-to-Leading order perturbative QCD analysis of world data on
the spin dependent structure functions $g_1^p, g_1^n$, and $g_1^d$, including
the new experimental information on the $Q^2$ dependence of $g_1^n$. Careful
attention is paid to the experimental and theoretical uncertainties. The data
constrain the first moments of the polarized valence quark distributions, but
only qualitatively constrain the polarized sea quark and gluon distributions.
The NLO results are used to determine the $Q^2$ dependence of the ratio
$g_1/F_1$ and evolve the experimental data to a constant $Q^2 = 5 GeV^2$. We
determine the first moments of the polarized structure functions of the proton
and neutron and find agreement with the Bjorken sum rule.
