  The COmmon Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS)
is a multi-purpose fixed-target experiment at the CERN Super Proton Synchrotron
(SPS) aimed at studying the structure and spectrum of hadrons. The two-stage
spectrometer has a good acceptance for charged as well as neutral particles
over a wide kinematic range and thus allows to access a wide range of
reactions. Light mesons are studied with negative (mostly $\pi^-$) and positive
($p$, $\pi^+$) hadron beams with a momentum of 190 GeV/$c$. The spectrum of
light mesons is investigated in various final states produced in diffractive
dissociation reactions at squared four-momentum transfers to the target between
0.1 and 1.0 $(\text{GeV}/c)^2$. The flagship channel is the $\pi^-\pi^+\pi^-$
final state, for which COMPASS has recorded the currently largest data sample.
These data not only allow to measure the properties of known resonances with
high precision, but also to search for new states. Among these is a new
resonance-like signal, the $a_1(1420)$, with unusual properties. Of particular
interest is also the resonance content of the partial wave with spin-exotic
$J^{PC} = 1^{-+}$ quantum numbers, which are forbidden for quark-antiquark
states.
