This article reviews the prevalence of gambling and related mental disorders from a public health perspective. It traces the expansion of gambling in North America and the psychological, economic, and social consequences for the public's health, and then considers both the costs and benefits of gambling and the history of gambling prevalence research. A public health approach is applied to understanding the epidemiology of gambling-related problems. International prevalence rates are provided and the prevalence of mental disorders that often are comorbid with gambling problems is reviewed. Analysis includes an examination of groups vulnerable to gambling-related disorders and the methodological and conceptual matters that might influence epidemiological research and prevalence rates related to gambling. The major public health problems associated with gambling are considered and recommendations made for public health policy, practice, and research.