Subsequent to the discovery of the X(3872) state, a number of exotic, charmonium(bottomonium)-like states referred to as the XYZ mesons have been observed in collider experiments including Belle, BaBar, BESIII, ... and LHCb. Properties of these states are inconsistent with the conventional quarkonium model and ,thus, the underlying nature of them remains still a mystery. We review properties of existing XYZ meson states and possible future work in this research field.