  I review the current status of the determination of helicity-dependent, or
polarized, parton distribution functions from a comprehensive analysis of
experimental data in perturbative quantum chromodynamics. I illustrate the
latest achievements driven by new measurements in polarized proton-proton
collisions at the Relativistic Heavy Ion Collider, namely the first evidence of
a sizable polarized light sea quark asymmetry and of a positive polarized gluon
distribution in the proton. I discuss which are the open issues in the
determination of polarized distributions, and how these may be addressed in the
future by ongoing, planned and proposed experimental programs.
