GAD has rates of comorbidity that equal or exceed those of other anxiety disorders, and it is one of the most common comorbid conditions with other disorders. Depressive disorders, especially MDD, and other anxiety disorders, especially panic disorder, most commonly co-occur. The pattern of comorbidity is consistent in community and clinical populations and in children and elderly people. Comorbidity is associated with greater impairment, more treatment seeking, and worse outcome among persons with GAD compared with pure GAD. Likewise, patients with panic disorder and MDD who have coexisting GAD tend to have more severe symptoms and less favorable outcome. The relationship between GAD and MDD seems especially close, and data from twin studies suggest that these conditions share a genetic diathesis. Patients with GAD and coexisting conditions respond less well to psychological and pharmacologic treatment, but, for those who do respond, treatment for the primary disorder often also produces improvement in comorbid conditions. Thus, research continues to show that GAD is important as a primary and a comorbid disturbance.