Compound and single-chambered eyes map the pattern of light in the optic array onto a sheet of light-sensitive receptor cells, so transforming it into a pattern of electrical activity in these cells. This pattern of receptor cell activity must in turn be transformed so that information needed to guide the animal’s actions is made available. These further transformations take place in the central nervous system, and one way of studying them is to record the electrical activity of single nerve cells in the retina, optic nerve, and brain in response to stimulation by light. The ultimate aim of this approach is to understand how information important to an animal is detected by networks of nerve cells and represented in patterns of neural activity. For all but the simplest animals, this is a distant goal indeed, and our knowledge does not yet extend beyond the early stages of neural transformation of patterns of light. In this chapter we describe the first of these stages, the transformation that the pattern of receptor cell activity undergoes in the retina. We consider first a relatively simple example, and then go on to the more complex retina of vertebrates.