Generalized anxiety disorder (GAD) is a chronic disturbance characterized by excessive worry and apprehension accompanied by psychic and somatic symptoms of stress and anxiety.A century ago, Freud noted that chronic, free-floating anxiety occurred frequently in the general population, and yet, to this day, there is limited information available about the natural history of this disorder.[1] Its resemblance to normal anxiety and the lack of distinctive distinguishing features have led to poor diagnostic reliability and questions about the validity of the disorder. The relative mildness of the symptoms and the high rate of comorbidity with other psychiatric disorders--the highest of all the anxiety disorders--have caused some to view it as an associated feature of a number of other disorders rather than as an independent disturbance. Nevertheless, no matter where the diagnostic threshold is set, GAD is common, and it is the least studied of the anxiety disorders. This article reviews the literature to date related to the natural history of GAD, including changes in diagnostic criteria over the past decade, as well as data to support the validity of the diagnosis, such as epidemiologic data, family studies, course of illness, and outcome studies.