  Quantum Chromodynamics (QCD), the theory of strong interactions, in principle
describes the interaction of quark and gluon fields. However, due to the
self-coupling of the gluons, quarks and gluons are confined into hadrons and
cannot exist as free particles. The quantitative understanding of this
confinement phenomenon, which is responsible for about 98\% of the mass of the
visible universe, is one of the major open questions in particle physics. The
measurement of the excitation spectrum of hadrons and of their properties gives
valuable input to theory and phenomenology.
  In the Constituent Quark Model (CQM) two types of hadrons exist: mesons, made
out of a quark and an antiquark, and baryons, which consist of three quarks.
But more advanced QCD-inspired models and Lattice QCD calculations predict the
existence of hadrons with exotic properties interpreted as excited glue
(hybrids) or even pure gluonic bound states (glueballs).
  The COMPASS experiment at the CERN Super Proton Synchrotron has acquired
large data sets, which allow to study light-quark meson and baryon spectra in
unprecedented detail. The presented overview of the first results from this
data set focuses in particular on the light meson sector and presents a
detailed analysis of three-pion final states. A new $J^{PC} = 1^{++}$ state,
the $a_1(1420)$, is observed with a mass and width in the ranges $m =
1412-1422\,\mathrm{MeV}/c^2$ and $\Gamma = 130-150\,\mathrm{MeV}/c^2$.
