A large number of exotic candidates, especially in the heavy sector,
have being observed in experiment, such as Zc(3900), Zc(4020), Pc(4380),
Pc(4450), X(5568) and so on. To understand their nature, numerous explanations
are proposed within different frameworks. I will give an overall review of the status of these studies and emphasize key issues to be answered by these solutions.
Crucial experimental observables are also proposed to distinguish them.