Anxiety is a common and normal phenomenon, involving multiple brain regions, including amygdala, locus ceruleus, and frontal cortex. Moreover, multiple brain transmitters regulate the presence and severity of anxiety; these include classical transmitters such as gamma aminobutyric acid, serotonin, and dopamine, as well as neuropeptides that include corticotrophin releasing hormone, substance P, neuropeptide Y, cholecystokinin, and vasopressin. Anxiety is highly adaptive, and involves both acute fear, related to an immediate threat, and anticipatory anxiety that is associated with possible future threat. Certain individuals are predisposed to develop anxiety disorders. Predisposing variables include both genetic factors (that may dispose toward anxious temperament), emotional traumas, and other psychologically mediated factors. Anxiety disorders represent a family of conditions with important distinguishing elements. Panic disorder and phobias involve reactions that are reminiscent of acute fear (albeit often worse). Specific and social phobias involve excessive fearful responses to identifiable things or circumstances in the environment. On the other hand, panic disorder is characterized by acute and intense fear responses that are not associated with a specific environmental cue, although people with this illness may experience aversive conditioning as a result of having spontaneous panic attacks in specific situations. Generalized anxiety disorder is a condition that, essentially, involves anticipatory fear (i.e., worry). Worries of everyday life are enhanced beyond any normal or adaptive functioning. Finally, acute stress disorder (ASD) and post-traumatic stress disorder (PTSD), while not technically classified as anxiety disorders, will be discussed in this chapter. These occur after a catastrophic, often life-threatening event. People with these conditions suffer persistent re-experiencing of the event (including intrusive thoughts or nightmares), avoidance of reminders of the event, and signs of increased arousal, such as difficulty sleeping, hypervigilance, or exaggerated startle response. Although complex, anxiety disorders are treatable conditions that respond to certain medications and specialized forms of psychotherapy.