The research of the past 20 years has markedly changed the image of fish. They are no longer viewed as a set of mechanical reflexes, but as cognitive beings. Fish exhibit flexible behaviour, have a good memory, and are able to learn and to cooperate. In addition to cognitive abilities, the consciousness of fish has also drawn recent attention, through the discovery that fish have the prerequisites for feeling pain. This has led to a debate about whether fish really do feel pain, and raises questions that are philosophical as well as biological: what is pain? What is consciousness? How can we tell whether a creature has conscious perception?