This chapter describes the epidemiology of co-occurring disorder (COD), and discusses its assessment. It suggests the use of simplified diagnostic criteria to confirm substance use disorder (SUD) in a patient with known or suspected serious mental illness (SMI), assess and treat the patient with known or suspected SMI for a concurrent drug intoxication. The chapter also discusses disposition of the COD patient who is no longer acutely intoxicated, withdrawing or suffering from an acute medical condition. In the emergency department (ED) setting, patients with potential or known COD typically present with acute behavioral disturbance. The Drug Abuse Screening Test Modified for ED (DAST-ED) is adapted for specific use in the ED and is based on two well-known drug abuse screening tests that have been well studied and validated for use in the outpatient setting. Psychosocial treatments shown to be effective include motivational interviewing, cognitive behavioral therapy, and social skills training.