Two photon width of heavy pseudoscalar mesons

We discuss the partial width of the pseudoscalar charmonium state eta_c and bottomonium state eta_b into two photons. Predictions from potential models are examined and compared with experimental values for the eta_c case. Through the NRQCD factorisation procedure results for eta_c are also compared with those from J/psi data, and results for eta_b to the Upsilon decay data.