Spin physics and the COMPASS experiment at CERN

The primary physics goals of the COMPASS experiment are the measurement of the gluon polarization in the nucleon and the determination of the longitudinal and transverse quark spin distribution functions at small x. In addition a rich program of hadron spectroscopy with hadron beams is foreseen. Experiments carried out at CERN, SLAC and DESY observed that the contribution of the valence quarks to the nucleon spin is small. SMC and HERMES have performed detailed measurements of the polarizations of the different quark flavors and found that the quark-sea contribution to the nucleon spin is compatible with zero. Recently predictions of lattice calculations and QCD-inspired models became available, which predict a sizeable contribution of the gluons to the nucleon spin. Therefore it has become a high priority to investigate the gluon polarization $ΔG/G.$ In COMPASS $ΔG/G$ will be studied via the photon-gluon fusion process, tagged either by open-charm production, or by high-$pT$ hadron pair production. An overview of the spin physics program and perspectives for COMPASS running in 2001 and beyond are presented.