The study of emotional states in animals is essential to our understanding of their welfare. Animals are sentient beings and experience both negative (e.g. fear, frustration) and positive (e.g. pleasure) emotions. In addition, there is large variability in emotional reactivity between animals, named temperament. A good understanding of an animal's emotions and its temperament traits will allow us to modify genetic selection and change housing practices to meet the welfare needs of animals. It is only very recently that the nature of emotions in animals and the way to assess their temperament have been under investigation. According to appraisal theories in psychology, an emotion results from the sequential evaluation of the triggering situation based on elementary characteristics (suddenness, unfamiliarity and pleasantness of the situation, individual ability to predict and to control it). For instance, an unfamiliar and unpleasant situation elicits either fear if the individual cannot control it or anger if the situation is under its control. Research in sheep has shown that animals use the same elementary characteristics as humans to evaluate their environment, allowing inference of their emotional experience. Cognitive processes are thus relevant to assess emotions in animals. In turn, repeated emotions can bias cognitive functions such as judgement and decision-making, which explain how temperament traits can be strongly expressed. Temperament in farm animals, such as sheep, cattle and quail, has a strong genetic basis. Most of the selection for temperament has been based on phenotypic parameters measured during behavioural tests. Heritability for some temperament traits is about 0.3, depending on tests and species, allowing efficient selection. Animal welfare may be thus improved by selecting animals with moderate levels of emotional reactivity to various eliciting events (human contact, novel situation, isolation). Moreover, selection for temperament has been associated with better reproductive rates and even increased meat quality.