  A study of the eta'->eta pi pi Dalitz plot distribution is presented in this
talk. The size of the branching ratio is properly understood within U(3) Chiral
Perturbation Theory and Resonance Chiral Theory, in the framework of the 1/Nc
expansion. Nonetheless, unitarity effects of pion loops must be incorporated in
order to achieve an appropriate description of the Dalitz slope parameters.
After taking the final state interactions into account, our predictions become
now in agreement with the available experimental measurements, although some
clear differences show up with respect to previous theoretical estimates.
