Photorealistic image synthesis is an important problem for such diverse fields as architecture, scientific visualization, simulation systems, lighting and industrial design, remote sensing from satellites and robots, and entertainment and advertising. An important, largely unexplored area of computer image generation is the simulation of weathering and its effects on appearance. Weathering results from the interaction of the environment with the materials in the world. Existing models of materials in computer graphics assume that the materials are in pristine condition. This research considers the development of computer graphics techniques for modeling and rendering weathered materials.