This article argues that the quality of diagnostic tools used to measure anxiety disorders in children and adolescents has improved enormously in the past few years. As a result, prevalence estimates are less erratic, understanding of comorbidity is increasing, and the role of impairment as a criterion for “caseness” is considered more carefully. Several of the instruments developed for epidemiologic research are now being used in clinical settings. Further integration of laboratory methods and clinical and epidemiologic ideas will benefit children with anxiety disorders and their families.