Anxiety is a normal phenomenon that represents an ‘alarm system’, which allows preparation of physical and psychological responses to a perceived threat or danger (the ‘fight-or-flight’ response). Anxiety is usually appropriate, short-lived and controllable. When anxiety is present inappropriately, and its symptoms are abnormally severe, persistent and impair physical, social or occupational functioning, an ‘anxiety disorder’ can be diagnosed. In this section, we summarise what is known about the aetiology and neurobiology of the anxiety disorders included in the International Classification of Diseases, 11th edition (ICD-11) classification of mental disorders: generalised anxiety disorder, panic disorder with or without agoraphobia, specific phobia, social anxiety disorder and separation anxiety disorder [1]. As there is significant overlap in the neurobiology of these disorders, we discuss the anxiety disorders as a whole, highlighting specific aspects where relevant. Characteristic features of the anxiety disorders are shown in Table 9.4.1.