This article summarizes epidemiologic evidence on insomnia, including the prevalence, incidence, and risk factors, as well as its course and consequences. Approximately 10% of the adult population suffers from an insomnia disorder and another 20% experiences occasional insomnia symptoms. Women, older adults, and people with socioeconomic hardship are more vulnerable to insomnia. Insomnia is often a chronic condition, with a 40% persistence rate over a 5-year period. Insomnia is a significant public health problem that should be addressed at the individual level with appropriate clinical care and at the population level with large-scale sleep health interventions.