The closely associated concepts of welfare and stress may be considered as opposites since welfare cannot be achieved under stress and vice versa. Stress was first considered as an unspecific response to any challenge taxing the organism's resources where the HPA axis plays a central role [Selye H. A syndrome produced by diverse nocuous agents. Nature 1936:32]. Along the same lines, welfare was considered as the state of an individual on a continuum between poor and good depending on the efforts required to adapt to the environment [Broom DM. Animal welfare: concepts and measurement. J Anim Sci 1991;69:4167-75]. However, these views cannot explain opposite results such as up- vs. down-regulation of the HPA axis and hypo- vs. hyper-behavioural reactivity under chronic stress. Later, it was shown that aversive situations trigger stress responses only if the individual perceives them as aversive. Mason [Mason JW. A re-evaluation of the concept of 'non-specificity' in stress theory. J Psychiatr Res 1971;8:323-33] suggested that the unspecificity of stress responses originates from a common emotion that produces them. Welfare has also been defined in terms of emotional states by Dawkins [Dawkins MS. Animal suffering, the science of animal welfare. London: Chapman and Hall Ltd.; 1980] and Duncan [Duncan IJH. Welfare is to do with what animals feel. J Agric Environ Ethics 1993;6:8-14]. Hence, both concepts are linked to mental states. Recent advances in psychology suggest that the very nature of an emotion results from a series of evaluations of the triggering situation that the individual makes based on criteria including novelty, predictability, controllability, and others [Scherer KR. Appraisal considered as a process of multi-level sequential checking. In: Scherer KR, Schorr A, Johnstone T, editors. Appraisal processes in emotion: theory, methods, research. New York: Oxford University Press; 2001. p. 92-120]. It is therefore suggested that the discrepancies found in the literature in terms of responses of the HPA axis or modification of behaviour under aversive conditions may stem from differences in the way a situation is evaluated. It is argued that stress comes from the animal's evaluation of the outcome of a situation, and that welfare is the state resulting from that evaluation.