Ever since Feinstein coined the term "comorbidity", referring to the presence of any additional coexisting ailment in a patient with a particular index disease (J Chronic Dis 23:455-468, 1970), aspects of the phenomenon have been extensively studied. The aims of this chapter are: (1) to summarize the evidence of psychiatric comorbidity in anxiety disorders from adult population-based studies; (2) to present findings from the National Comorbidity Survey Replication (NCS-R); (3) to summarize evidence of psychiatric comorbidity in anxiety disorders from child and adolescent population-based samples; (4) to provide a summary of evidence on comorbidity from family and genetic studies; and (5) to examine patterns of comorbidity between anxiety disorders and medical conditions. Throughout each of these aims, implications of the comorbidity are explored, including whether these patterns reflect a need for redefining the disorders or rather an etiologic or even causal path.