This chapter describes the magnitude of the problem of mental illness, both globally and in terms of specific mental health-related visits encountered in emergency department (ED) settings. The WHO's cross-national comparisons show a globally high prevalence of major Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) mental disorders (anxiety disorders, mood disorders, impulse control disorders, substance use disorders) with 25th-75th percentiles ranging from 18.1% to 36.1%. The chapter describes the magnitude of the problem of ED presentations for specific mental disorders. The most prevalent conditions are highlighted. After anxiety disorders, mood disorders are the second most common psychiatric disorder in the general population, occurring in 10% of the U.S. adult population each year. Schizophrenia spectrum diagnoses account for approximately two thirds of all psychotic disorders. Almost 1 in 10 of the adult U.S. population is estimated to have an Axis II personality disorder in any year.