One in 20 U.S. adults aged 18 to 65, roughly 11 million people, had both a mental disorder and a substance use disorder in the same year, according to the Mental and Substance Use Disorders Prevalence Study. Many of them are told to fix one problem first and come back for the other later. That rarely works. Dual diagnosis residential treatment puts addiction and mental health under one roof and one plan, so neither condition gets left behind. This guide explains what dual diagnosis residential treatment involves, how it compares with other settings, and what to ask before choosing a program.