  The COMPASS experiment is a fixed target experiment at the CERN SPS using
muon and hadron beams for the investigation of the spin structure of the
nucleon and hadron spectroscopy. The main objective of the muon physics program
is the study of the spin of the nucleon in terms of its constituents, quarks
and gluons. COMPASS has accumulated data during 6 years scattering polarized
muons off a longitudinally or a transversely polarized deuteron (6LiD) or
proton (NH3) target. Results for the gluon polarization are obtained from
longitudinal double spin cross section asymmetries using two different
channels, open charm production and high transverse momentum hadron pairs, both
proceeding through the photon-gluon fusion process. Also, the longitudinal spin
structure functions of the proton and the deuteron were measured in parallel as
well as the helicity distributions for the three lightest quark flavors. With a
transversely polarized target, results were obtained with proton and deuteron
targets for the Collins and Sivers asymmetries for charged hadrons as well as
for identified kaons and pions. The Collins asymmetry is sensitive to the
transverse spin structure of the nucleon, while the Sivers asymmetry reflects
correlations between the quark transverse momentum and the nucleon spin.
Recently, a new proposal for the COMPASS II experiment was accepted by the CERN
SPS which includes two new topics: Exclusive reactions like DVCS and DVMP using
the muon beam and a hydrogen target to study generalized parton distributions
and Drell-Yan measurements using a pion beam and a polarized NH3 target to
study transverse momentum dependent distributions.
