This chapter discusses the evidence for the role of genetic factors in the etiology of anxiety disorders, and summarizes the genetic study designs used in research on anxiety disorders. Molecular genetic study designs used to investigate the genetics of anxiety disorders include linkage analysis and candidate gene association studies. Twin studies support a heritability estimate between 30% and 40%. More recently, regulators of G-protein signaling have been investigated regarding anxiety-related phenotypes including panic disorder. Family studies suggest that risk of social anxiety disorder (SAD) to first-degree relatives of SAD probands ranges from 16% to 26%. Investigations of panic disorder, specific phobias, SAD, and obsessive-compulsive disorder (OCD) have produced some evidence of linkage to specific regions. Two exciting yet mostly unexplored areas in anxiety disorder research are gene-environment interaction and epigenetic studies. Epigenetic research examines the dynamic heritable changes in the function of a gene.