  The XYZ mesons are unexpected mesons containing a heavy quark-antiquark pair
that have been discovered during the last decade. The models for the XYZ mesons
that have been proposed include conventional quarkonium, quarkonium hybrids,
and quarkonium tetraquarks, whose four constituents can be clustered in several
possible ways. None of the models have provided a compelling pattern for the
XYZ mesons. The most promising theoretical approaches within QCD are lattice
QCD for the c c-bar mesons, lattice NRQCD for the b b-bar mesons, and the
Born-Oppenheimer approximation. The additional hints that will be provided by
future experiments guarantee the eventual solution of the XYZ puzzle.
