A review of the chain of experiments whose results were finally explained by Compton's discovery of the Compton effect shows that these experiments were not solely in the field of x rays. The studies of the scattering of high-energy gamma rays of radium had presented physicists with the puzzle that scattered gamma rays were distinctly less penetrating than were those that had not been scattered, and that the amount of this “softening” of the scattered gamma rays was dependent on both the initial “hardness” of the gamma rays and on the scattering angle. Many of the experimental facts of the Compton effect were well known from the studies of scattering of gamma rays some years before the effects were observed with x rays. Compton's interest in the problem was apparently greatly stimulated by his own experiments on the scattering of gamma rays.However, since the first detailed and convincing experimental verification of several of the consequences of Compton's hypothesis came from experiments with x rays, one may not be aware of the important role played by gamma-ray experiments in the steps leading to the discovery.