The slaughter of animals comprises the induction of unconsciousness followed by bleeding to cause death. Today’s slaughter practices are chosen to avoid animal suffering, but what does science tell us about animal suffering? Do animals have emotions? Consciousness? How to study consciousness? Experiments suggest strongly that animals have emotions and are conscious, although many aspects of consciousness are still not understood. However, various brain areas involved in consciousness have been identified and the mechanical, electrical and gaseous stunning techniques used at slaughter, cause dysfunction of one or several of these areas, in different manners to induce unconsciousness.