Everyone experiences, at least on occasion, feelings of anxiety—for example, pangs of dread before giving a public speech, worries about relationships or success at work, or a swelling sensation of fear when walking down a dimly lit street. These common experiences, however, are qualitatively different from clinical anxiety disorders, which are more than just exaggerated manifestations of these common experiences. Rather, individuals with anxiety disorders are burdened frequently by invasive thoughts and overwhelming emotions that can come on without warning, cause severe personal distress, and interfere with an individual's day-to-day functioning and personal relationships. As many as 40 million adults in the United States suffer from clinical anxiety disorders, which fall primarily into six diagnostic categories: panic disorder (PD), generalized anxiety disorder (GAD), social phobia or social anxiety disorder (SAD), specific phobia, posttraumatic stress disorder (PTSD), and obsessive-compulsive disorder (OCD).