Light is necessary for the human visual system to operate. With light, we can see; without light, we cannot. This chapter describes the structural, operational and perceptual characteristics of the human visual system. The first thing to appreciate about the visual system is that it is not the eye alone. Rather, it is the eye and brain working together. The visual system is often likened to a camera but this analogy is misleading. The only parts of the visual system that resemble a camera are the optical components of the eye. Once the optical components have formed an image of the world on the retina, the camera analogy fails.