This paper presents a method for the efficient realistic shading of cumulus clouds. Our approach consists in taking advantage of all the a priori knowledge on the characteristics of such objects and on the light effects that are known to occur, thus avoiding a time consuming (blind) physical simulation of multiple scattering through volumes. This paper only focuses on cloud shading. It does not deal with the modeling and animation of the cloud's shape, or with sky models. The features we take into account are the illumination, including the effects of the inter-reflections and of the environment, and the cloud's corona behavior, especially when illuminated from the back. We show some results obtained with our preliminary implementation, illustrating these features. All the images are ray-traced in about 1 1/2 minute on a SGI O2.