Repetitive behaviour can have various causes, many of which are medical. However, even repetitive behaviours that are not based on an identifiable medical condition vary as to their aetiology: they may be a normal part of the species-typical repertoire, they may be conditioned, or they may be environmentally induced abnormal behaviours. This chapter looks at causes and risk factors, evaluation of the patient, diagnosis, behavioural biology of the condition, treatment, prognosis and follow-up.