Selective breeding programmes in domestic and laboratory animals generally focus on physiological and/or anatomical characteristics. However, selection may have an (unintended) impact on other characteristics and may lead to dysfunctional behaviour that can affect biological functioning and, as a consequence, compromise welfare and quality of life. In this review it is proposed that various behavioural dysfunctions in animals are due to pathological anxiety. Although several approaches have been undertaken to specify the diagnostic criteria of pathological anxiety as a behavioural disorder in animals, the causal aetiology largely remains unknown. This is mainly due to the fact that integrated concepts, combining the behavioural syndrome and (neuro-) physiological processes, are widely lacking. Moreover, even the term anxiety itself represents a poorly defined concept or category. A definition is suggested and the potential causes of pathological anxiety are explored with a plea for developing adequate diagnostic tools and therapies to fight pathological anxiety in animals based on insight from scientific research.