  The COMPASS experiment at CERN's SPS investigates the structure and
excitations of strongly interacting systems. Using reactions of 190 GeV/c pions
with protons and nuclear targets, mediated by the strong and electromagnetic
interaction, an unprecedented statistical precision has been reached allowing
new insight into the properties of light mesons. For the first time the
diffractively produced 3pi final state has been analyzed simultaneously in bins
of invariant mass and four-momentum transfer using a large set of 88 waves up
to a total angular momentum of 6. In addition to a precise determination of the
properties of known resonances and including a model-indepedent analysis of the
pi pi S-wave isobar, a new narrow axial-vector state coupling strongly to
f0(980)pi has been found in previously unchartered territory. By selecting
reactions with very small four-momentum transfer COMPASS is able to study
processes involving the exchange of quasi-real photons. These provide clean
access to low-energy quantities such as radiative couplings and
polarizabilities of mesons, and thus constitute a test of model predictions
such as chiral perturbation theory.
