Polarised Structure Functions and Quark Distributions at Hera

In 1995, the HERMES experiment 1],,2] at HERA/DESY started its extensive program to study the spin structure of the nucleon through deep inelastic scattering (DIS) of longitudinally polarised positrons oo polarised ? ! H; ? ! D and 3 ? ! He targets. Inclusive polarised DIS measurements aim at a precise determination of the spin structure functions g 1 and g 2 of the proton and the neutron, using longitudinally as well as transversely polarised targets. Moments of these structure functions are computed and compared to predictions from sum rules. The primary contribution from HERMES to the investigation of the nucleon spin structure originates from the semi-inclusive polarised DIS measurements, taking advantage of the unique capability of the experiment to identify nal state hadrons. These measurements allow access to the spin momentum distributions of the various quark avours and their decomposition into valence and sea. After the upgrades to the experiment in 1998, HERMES will be able in addition to separate the nucleon spin contributions from the strange and non-strange sea. Moreover, HERMES intends to make a worldwide rst`direct' measurement of the gluon polarisation via the detection of charm decays.