  Semi-inclusive hadron production in longitudinally polarized deep-inelastic
lepton-nucleon scattering is a powerful tool for resolving the quark flavor
decomposition of the proton's spin structure. We present the full
next-to-next-to-leading order (NNLO) QCD corrections to the coefficient
functions of polarized semi-inclusive deep-inelastic scattering (SIDIS) in
analytical form, enabling the use of SIDIS measurements in precision studies of
the proton spin structure. The numerical impact of these corrections is
illustrated by a comparison with data of polarized single-inclusive hadron
spectra from the DESY HERMES and CERN COMPASS experiments.
