The Singapore Mental Health Study (SMHS) is a population-based, cross-sectional, epidemiological study on the Singapore multi-ethnic adult population. This article provides an overview of the research design and methods used which took into consideration the unique characteristics of the country and its multi-ethnic population. A face-to-face household survey of Singapore residents aged 18 years and above was undertaken from 2009 to 2010. The nationally representative probability sample was derived using a disproportionate stratified sampling method. In order to increase precision for subgroup estimations the design was stratified with over-sampling of Malays, Indians and those aged 65 years and above. Respondents were assessed using the English, Chinese (computerized) and Malay (paper and pencil based) version of the Composite International Diagnostic Interview (CIDI) 3.0 to establish lifetime and 12-month prevalence of mental disorders, the current use of mental health services (both Western and traditional services), the treatment gaps and loss of role functioning.