  In recent years there has been a rapidly growing body of experimental
evidence for existence of exotic, multiquark hadrons, i.e. mesons which contain
additional quarks, beyond the usual quark-antiquark pair and baryons which
consist of more than three quarks. In all cases with robust evidence they
contain at least one heavy quark Q=c or b, the majority including two heavy
quarks. Two key theoretical questions have been triggered by these discoveries:
(a) how are quarks organized inside these multiquark states -- as compact
objects with all quarks within one confinement volume, interacting via color
forces, perhaps with an important role played by diquarks, or as deuteron-like
hadronic molecules, bound by light-meson exchange? (b) what other multiquark
states should we expect? The two questions are tightly intertwined. Each of the
interpretations provides a natural explanation of parts of the data, but
neither explains all of the data. It is quite possible that both kinds of
structures appear in Nature. It may also be the case that certain states are
superpositions of the compact and molecular configurations. This Whitepaper
brings together contributions from many leading practitioners in the field,
representing a wide spectrum of theoretical interpretations. We discuss the
importance of future experimental and phenomenological work, which will lead to
better understandingof multiquark phenomena in QCD.
