Evidence that panic and generalized anxiety disorder are separate disorders is presented from a review of epidemiological, family, and twin studies. Generalized anxiety disorder and panic disorder have been shown to have different rates and risk factors in the community and different family aggregation, which is consistent with the separation of the disorders. The one small twin study using Diagnostic and Statistical Manual, Third Edition criteria also supports the separation between these disorders. The concordance for panic disorder is greater among monozygotic than among dizygotic twins. There undoubtedly are overlaps between these disorders, and some patients with generalized anxiety disorder may later develop panic disorder; however, the results thus far support the separation of these disorders.