This Treatment Improvement Protocol (TIP) update is intended to provide addiction counselors and other providers, supervisors, and administrators with the latest science in the screening, assessment, diagnosis, and management of co-occurring disorders (CODs). For purposes of this TIP, CODs refer to co-occurring substance use disorders (SUDs) and mental disorders. Clients with CODs have one or more disorders relating to the use of alcohol or other substances with misuse potential as well as one or more mental disorders. A diagnosis of CODs occurs when at least one disorder of each type can be established independent of the other and is not simply a cluster of symptoms resulting from the one disorder. Many may think of the typical person with CODs as having a serious mental illness (SMI) combined with a severe SUD, such as schizophrenia combined with alcohol use disorder (AUD).