To present an overview of the World Health Organization World Mental Health (WMH) Survey Initiative. The discussion draws on knowledge gleaned from the authors' participation as principals in WMH. WMH has carried out community epidemiological surveys in more than two dozen countries with more than 200,000 completed interviews. Additional surveys are in progress. Clinical reappraisal studies embedded in WMH surveys have been used to develop imputation rules to adjust prevalence estimates for within- and between-country variation in accuracy. WMH interviews include detailed information about sub-threshold manifestations to address the problem of rigid categorical diagnoses not applying equally to all countries. Investigations are now underway of targeted substantive issues. Despite inevitable limitations imposed by existing diagnostic systems and variable expertise in participating countries, WMH has produced an unprecedented amount of high-quality data on the general population cross-national epidemiology of mental disorders. WMH collaborators are in thoughtful and subtle investigations of cross-national variation in validity of diagnostic assessments and a wide range of important substantive topics. Recognizing that WMH is not definitive, finally, insights from this round of surveys are being used to carry out methodological studies aimed at improving the quality of future investigations.