Mental disorders among children are described as "serious deviations from expected cognitive, social, and emotional development". These disorders are an important public health issue in the United States because of their prevalence, early onset, and impact on the child, family, and community, with an estimated total annual cost of $247 billion. A total of 13%–20% of children living in the United States experience a mental disorder in a given year, and surveillance during 1994–2011 has shown the prevalence of these conditions to be increasing. 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 behaviours (e.g., alcohol and drug abuse) and chronic conditions, and use of mental health-related care and clinical services. This book summarises data from selected CDC surveillance systems that measure the prevalence and impact of mental illness in the U.S. child and adult populations.