Institut de recherche sur les lois fondamentales de l'Univers

The recent observations of XYZ heavy quarkonium states have initiated a lot of interests in the heavy quarkonium spectroscopy. In particular, the observations of signals for charged heavy quarkonium states, such as Z_b(10610) and Z_b(10650) by the Belle Collaboration, and Z_c(3900) by the BESIII Collaboration, provide a great opportunity for the study of QCD exotics.  I will review some of the theoretical explanations for their puzzling properties, and then discuss an important kinematic condition in favor of the production of some of those "threshold states" in heavy quarkonium decays. Criteria for further identifying exotic states will also be discussed.