This course presents a myriad of co-occurring disorders that occur in the population of people with substance use disorders and offers a brief description of selected mental disorders and their DSM-5 diagnostic criteria, as well as the descriptive and diagnostic features, prevalence statistics, and relationship to SUDs. There are strategies for working with such individuals, basic approaches that can support the therapeutic relationship and make interventions more effective, as well as the competencies needed for those in addiction counseling. This course is chapters 4 and 5 of Substance Use Disorder Treatment for People With Co-Occurring Disorders, Treatment Improvement Protocol (TIP) Series, No. 42. This treatment protocol is offered by the Substance Abuse and Mental Health Services Administration (SAMHSA) and was updated in March 2020.