  The SU(3)_flavor constituent quark model has been quite successful to explain
the properties as well as the observed spectrum of mesons with pseudoscalar and
vector quantum numbers. Many radial and orbital excitations of quark-antiquark
systems predicted by the model, however, have not yet been observed
experimentally or assigned unambiguously. In addition, a much richer spectrum
of mesons is expected from QCD, in which quarks interact which each other
through the exchange of colored self-interacting gluons. Owing to this
particular structure of QCD, configurations are allowed in which an excited
gluonic field contributes to the quantum numbers J^{PC} of the meson. States
with a valence color-octet qqbar' pair neutralized in color by an excited gluon
field are termed hybrids. The observation of such states, however, is difficult
because they will mix with ordinary qqbar' states with the same quantum
numbers, merely augmenting the observed spectrum for a given J^{PC}. Since the
gluonic field may carry quantum numbers other than 0^{++}, however, this can
give rise to states with "exotic" quantum numbers J^{PC}=0^{--}, 0^{+-},
1^{-+}, 2^{+-},... The lowest-lying hybrid multiplet is expected to contain a
state with exotic quantum numbers J^{PC}=1^{-+}. The identification of such a
state is considered a "smoking gun" for the observation of non-qqbar mesons.
The search for hybrid states has been a central goal of hadron spectroscopy in
the last 20 years. Ongoing and upcoming high-statistics experiments are
expected to shed new light on the existence of such states in nature. In this
paper, theoretical predictions for masses and decay modes as well as recent
experimental evidence for hybrid meson states and future experimental
directions are discussed.
