Childhood anxiety disorders are very common, carry tremendous personal and societal costs, frequently do not respond adequately to treatment, and involving parents in treatment has so far not enhanced outcomes. Explanatory clinical trials are needed to identify parent specific mechanisms of change that are not targeted in direct child treatment, and to identify markers of who is most likely to benefit from parent intervention. This study is an explanatory clinical trial of a parent based intervention and of cognitive behavioral therapy, and an investigation of biological and behavioral moderators of treatment response.