The literature reviewed here is consistent in showing that GAD is a common mental disorder that typically has an early age of onset, a chronic course, and a high degree of comorbidity with other anxiety and mood disorders. Comorbid GAD is often temporally primary, especially in relation to mood disorders, and is associated with an increased risk for the subsequent onset and severity of secondary disorders. The weight of evidence reviewed here argues against the view expressed by early commentators that GAD is better conceptualized as a prodrome, residual, or severity marker of other disorders than as an independent disorder. Focused studies of comorbidity between GAD and major depression, in which comorbidity is high, lead to the same conclusion. The crucial evidence for this conclusion includes the following: 1. Contrary to the findings of clinical studies, GAD in the community does not have a higher comorbidity than do most other anxiety or mood disorders. 2. The symptoms of GAD form an empiric cluster distinct from the symptoms of major depression in studies of symptom profiles. 3. Family studies show distinct aggregation of GAD and major depression. 4. Twin studies show that the environmental determinants of GAD are different from the environmental determinants of major depression. 5. The sociodemographic predictors of GAD in epidemiologic studies are different from the predictors of major depression. 6. The clinical course of GAD is less consistently related to comorbidity than is the course of other anxiety and mood disorders. 7. The impairments associated with GAD are equivalent to, or greater than, those associated with other severely impairing chronic physical and mental disorders. These findings show that the status of GAD as an independent disorder is at least as strongly supported by available evidence as is that of other anxiety or mood disorders. This article also shows that uncertainty remains regarding even the basic epidemiologic characteristics of the GAD syndrome. Lingering concerns about the independence of GAD have conspired to exacerbate this problem by promoting repeated changes in the diagnostic criteria for GAD from the DSM-III to DSM-III-R and to DSM-IV. These successive changes have made it difficult to amass consistent long-term data on the natural history of GAD. Available evidence on the empiric validity of current diagnostic thresholds for GAD raises questions about the requirements, such as whether a 6-month minimum duration and four or more additional psychophysiologic symptoms are optimal for identifying all of the people in the general population who suffer from a clinically significant GAD syndrome. An additional source of potential bias in this regard is that the DSM system requires that anxiety be excessive or unrealistic for a diagnosis of GAD. Interestingly, there is no comparable DSM requirement that dysphoria must be excessive or unrealistic to qualify as major depression. These diagnostic uncertainties make it difficult to gain a clear understanding of the true breadth and depth of the GAD syndrome in the general population. Additional research is needed, ideally from unbiased epidemiologic samples, to resolve these basic uncertainties. The strong comorbidity between GAD and major depression, the fact that most people with this type of comorbidity report that the onset of GAD occurred before the onset of depression, and the fact that temporally primary GAD significantly predicts the subsequent onset of depression and other secondary disorders raise the question of whether early intervention and treatment of primary GAD would effectively prevent the subsequent first onset of secondary anxiety and depression. Unfortunately, little is known about this possibility because, as mentioned earlier, few people with pure GAD seek treatment. Why this is true is unknown. Given the early onset of GAD and its strong effects in predicting the subsequent onset, severity, and persistence of other disorders, efforts are needed to collect epidemiologic data on the reasons for the low rate of help seeking among people with pure GAD and to develop outreach strategies that may correct this situation.