The purpose of this study is to evaluate the safety, tolerability, and effectiveness of SSRI and SNRI medications for children and adolescents with anxiety disorders, depressive disorders, eating disorders, or obsessive-compulsive disorder. The study will characterize predictors of outcome, including demographic, disease severity, comorbidity, concomitant treatment, and genetic variation. This information will help clinicians to better understand the balance of risk and benefit associated with antidepressants and to answer the question of which treatment is best for which child.