  The nucleon spin structure is studied at COMPASS via the measurement of the
spin dependent properties of deep inelastic muon-deuteron scattering. Both
inclusive and semi-inclusive observables are investigated, with the emphasis
put on the latter. This allows to access, in particular, the gluon polarization
($\Delta G/G$) and the flavor decomposition of the quark helicity
distributions. Results from measurements with longitudinal target polarization
are presented, as well as uncertainties expected from the first 3 years of data
taking.
