Some models for the decision-making processes in animal behaviour are discussed, together with their merits and drawbacks. Abnormal behaviour is defined as an untypical reaction to a particular combination of motivational factors and stimuli. Much abnormal behaviour can be regarded as stress-coping responses. Three categories of abnormalities are discussed: stereotyped motor patterns, abnormal aggression and cannibalism. It is concluded that animals with high frequencies of abnormal behaviour should be regarded as not healthy and the experimental results treated in accordance with this. Methods of preventing abnormal behaviour consist of the recognition of the species-specific ethological demands and the incorporation of these demands into the housing systems of the animals.