Mental illnesses account for a larger proportion of disability in developed countries than any other group of illnesses, including cancer and heart disease. In 2004, an estimated 25% of adults in the United States reported having a mental illness in the previous year. The economic cost of mental illness in the United States is substantial, approximately $300 billion in 2002. Population surveys and surveys of health-care use measure the occurrence of mental illness, associated risk behaviors (e.g., alcohol and drug abuse) and chronic conditions, and use of mental health–related care and clinical services. Population-based surveys and surveillance systems provide much of the evidence needed to guide effective mental health promotion, mental illness prevention, and treatment programs.