The equations used to compute shading are known as a shading model, and a range of different shading models have been developed for these different applications. Generally, they all begin with simple models that provide a useful approximation to the physics of light reflection. This chapter describes a basic shading model for an opaque surface illuminated by a point light source. Ideal diffuse shading, often called Lambertian shading because Lambert’s cosine law is the main effect it models, provides a flat, chalky appearance by itself. Since specular reflection is view dependent, it is a function of the view vectorv that points from the shading point toward the viewer, as well as the normal vector n and light direction l. The shading calculation then boils down to computing the irradiance and the bidirectional reflectance distribution function, and then multiplying them.