Gamma-gamma partial widths of charmonium states. An overview of recent and not so recent data on the partial widths of the two-photon decay channels of charmonium states is given. In particular, data from pbar-p and e+e- colliders are compared, and peculiarities of the results are discussed. A measurement of the eta-c formfactor is shown, as well as some upper limits on the formation of the eta-c'.