Anxiety is a common and normal phenomenon, involving multiple brain regions, including the 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, including corticotrophin-releasing hormone, substance P, neuropeptide Y, cholecystokinin, and vasopressin. Anxiety is highly adaptive, and involves both acute fear that is related to an immediate threat, and anticipatory anxiety that is associated with a possible future threat. Certain individuals seem predisposed to develop anxiety disorders. Predisposing variables include genetic factors (that may function to 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, exclusively involves anticipatory fear (i.e., worry). Worries of everyday life are enhanced beyond any normal or adaptive functioning. Obsessive-compulsive disorder subsumes both obsessive ideation and compulsive behaviors. Obsessions are fears or worries that are fixated on unlikely events, and cannot be reduced by normal reassurance. Compulsions are repetitive behaviors that typically are in response to an obsession. For example, a person may have a fear of contamination that might lead to a terrible disease in themselves or someone else. This, then, leads to compulsive hand washing that may occur dozens of times per day. Although the person usually recognizes that the fear is excessive, they have difficulty controlling the worry. Finally, acute and posttraumatic stress disorders occur after a catastrophic, usually life-threatening event. People with these conditions experience persistent reexperiencing 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. © 2008 Humana Press, a part of Springer Science+Business Media, LLC.