On its p a th from a light source to the cam era, a light ray is altered in m any characteristic ways by the objects in the scene. The cam era then encodes the m easured light into a color pixel. The goal of image understanding is to use properties and relationships between pixels to interpret the image. For such m ethods to be successful, it is essential th a t the reflection processes in the scene be understood and modeled, as well as the sensing characteristics of the cam era. This chapter describes the processes by which light interacts with objects in the scene.