Stereotypies are repetitive actions that are fixed in form and orientation and serve no obvious purpose. Their occurrence in farm animals submitted to intensive husbandry has been interpreted to indicate inadequate environmental design or welfare. Over recent years, detailed descriptive studies have led to a better understanding of the organization and development of stereotypies, particularly in pregnant sows housed in tether stalls. Stereotyped behaviors appear to emerge from elements of initial defensive reactions of animals to their environment. They develop via a progressive narrowing of the behavioral repertoire, until only self-directed behaviors are displayed. Pharmacological studies suggest that performance of stereotyped behavior depends on the brain dopamine systems that are involved in selection and initiation of motor movements. However, generalization of these data to natural stereotypies is doubtful. Recent evidence favors an involvement of brain opioid peptides in stereotyped behavior of sows, although the exact mechanisms remain unknown. Stereotypies are typically observed in situations of conflict or frustration. They have been claimed to have an adaptive function, either by providing self-generated sensory stimulation to compensate for the lack of environmental stimulation, or by allowing animals to discharge their tension or anxiety. However, there is little evidence in favor of either interpretation. On the basis of present knowledge, it is proposed that stereotyped activities gain strength because of the positive feedback effect of sensory stimulation on their underlying control systems, which leads to a progressive sensitization of these neural systems. Stereotypies would then be simply the outward expression of an activation of lower brain structures controlling motor behavior which occurs in the absence of normal inhibitory control by higher nervous functions.