This chapter discusses the results of the World Mental Health (WMH) surveys regarding the burden of mental disorders. The mental disorders were assessed with Composite International Diagnostic Interview (CIDI), a fully structured lay-administered interview designed to generate research diagnoses of commonly occurring mental disorders according to the definitions and criteria of both the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) and International Classification of Diseases (ICD-10) Diagnostic Criteria for Research (DCR) diagnostic systems. Generalized linear modeling (GLM) was used to assess the effects of mental disorders on continuous outcome variables with skewed distributions. The CIDI included retrospective disorder age-of-onset (AOO) reports based on a special question sequence that has been shown experimentally to improve recall accuracy. Mental disorders are important causes of productivity loss and low perceived health. They are among the most important disorders at both individual and societal levels.