Can a computer really be programmed to do such things? The field of computer vision deals with methods a computer can use to obtain information about objects and events in a scene by analysing images of the scene. These methods need not resemble those used by humans (or animals) to see the world as long as they yield correct results. In this chapter, we first discuss how a computer might distinguish objects in a scene -- for example, how it might distinguish a human head from its background or differentiate parts of the head (e.g., hair, eyes) from each other. We then explore how information about the parts can be used for recognition -- for example, in identifying a particular person's face in a collection of images. Finally, we consider how variations in the appearance of parts of a face over a sequence of images can be used to recognize a person's facial expressions.