  The distribution of the spin of the nucleon among its constituents can be
parametrized in the form of polarized parton distribution functions for quarks
and gluons. Using all available data on the polarized structure function
$g_1(x,Q^2)$, we determine these distributions both at leading and
next-to-leading order in perturbation theory. We suggest three different,
equally possible scenarios for the polarized gluon distribution, which is found
to be only loosely constrained by current experimental data. We examine various
possibilities of measuring polarized parton distributions at future
experiments.
