BACKGROUND: Since the introduction of specified diagnostic criteria for mental disorders in the 1970s, there has been a rapid expansion in the number of large-scale mental health surveys providing population estimates of the combined prevalence of common mental disorders (most commonly involving mood, anxiety and substance use disorders). In this study we undertake a systematic review and meta-analysis of this literature. METHODS: We applied an optimized search strategy across the Medline, PsycINFO, EMBASE and PubMed databases, supplemented by hand searching to identify relevant surveys. We identified 174 surveys across 63 countries providing period prevalence estimates (155 surveys) and lifetime prevalence estimates (85 surveys). Random effects meta-analysis was undertaken on logit-transformed prevalence rates to calculate pooled prevalence estimates, stratified according to methodological and substantive groupings. RESULTS: Pooling across all studies, approximately 1 in 5 respondents (17.6%, 95% confidence interval:16.3-18.9%) were identified as meeting criteria for a common mental disorder during the 12-months preceding assessment; 29.2% (25.9-32.6%) of respondents were identified as having experienced a common mental disorder at some time during their lifetimes. A consistent gender effect in the prevalence of common mental disorder was evident; women having higher rates of mood (7.3%:4.0%) and anxiety (8.7%:4.3%) disorders during the previous 12 months and men having higher rates of substance use disorders (2.0%:7.5%), with a similar pattern for lifetime prevalence. There was also evidence of consistent regional variation in the prevalence of common mental disorder. Countries within North and South East Asia in particular displayed consistently lower one-year and lifetime prevalence estimates than other regions. One-year prevalence rates were also low among Sub-Saharan-Africa, whereas English speaking counties returned the highest lifetime prevalence estimates. CONCLUSIONS: Despite a substantial degree of inter-survey heterogeneity in the meta-analysis, the findings confirm that common mental disorders are highly prevalent globally, affecting people across all regions of the world. This research provides an important resource for modelling population needs based on global regional estimates of mental disorder. The reasons for regional variation in mental disorder require further investigation.