The welfare of an animal is fulfilled when it can adapt to the environment. This will be the case if the environment is ideal so that all the needs are satisfied or if the environment is not ideal but that the animal can find a way to adapt whithout suffering or exhaustion. Anthropomorphism ought to be avoided in evaluating animal welfare. The role of the scientist is to determine objective measurements of adaptation. These consist of indices of health status, production parameters, neuro-endocrine responses to stress and behavioural deviations. Behavioural indices are very helpful due to their high sensitivity and specificity. In addition, the magnitude of behavioural alteration often varies with the severity of the constraint. Behavioural indices can be used as predictors (the circadian rhythm of activity is modified before clinical symptoms of a disease) and to detect mild variations between life conditions (the reduction of social contacts enhances behavioural reactivity without physiological change). Behavioural indices can give information on the nature of constraints. For example, lying postures of calves can be used to assess the lack of space and oral non-nutritive activities can be used to assess feeding systems. In some cases, there is a close relation between the frequency of some activities or postures and the severity of the constraint. This relation is more easily highlighted when synthetic behavioural scores are calculated. However, to choose between several rearing conditions, one should rely not only on behaviour but on a range of indices. These aspects are illustrated with examples on calves, the most striking constraints upon veal calves being restriction of place, absence of solid food and social isolation.