The diffuse radiation incident on an inclined plane is composed of both the ground-reflected radiation and the sky diffuse radiation. The evaluation of the sky diffuse radiation has already been described in many references. In this paper we focus on the ground-reflected radiation, its relation to insolation conditions and its evaluation by means of models. We used six data banks from the following four countries: Switzerland, France, The Netherlands, and the U.S.A. We investigated how the albedo depends on the amount and the composition of the incident radiation, on geometrical parameters such as the height and/or the azimuth of the sun and on meteorological parameters such as the humidity. We did not find any notable dependence. We also tested different models evaluating the ground-reflected radiation on tilted planes with corresponding measurements on an inverse horizontal plane (facing the ground) and on inclined planes. We came to the conservative conclusion that the best results are obtained when using a constant averaged measured albedo, for transposition to tilted surfaces, when assuming the ground-reflected radiation to be isotropic.