Imagery is a very common experience in everyday life: we hear a sound, and we imagine what might be its source, we try to remember where we have left our keys, and we see, with the mind's eye, all the actions we have done and the rooms where we have entered before going out; we are moving into a new house and we imagine the disposition of the furniture, the colours of the walls and so on; we have an examination, and we imagine the scene, the questions, our reactions. We use imagery to remember, to anticipate, to answer a question or to invent something new; but for many years psychology has neglected this problem because of the difficulty to find objective methods which were not based on introspection. Only recently the cognitivist approach has experienced techniques to study mental representations, proposing models that permit experimentation and computation: twenty years ago we had already good models of propositional representations, but none of other formats like image.