This chapter introduces the practitioners to substance use disorders in geropsychiatric populations, identifying criteria for diagnosis, assessments and evaluating co-morbidities, including differential diagnoses and indications for treatment options. It discusses the differences in addiction, dependence, tolerance, intoxication, substance withdrawal and treatment options for geropsychiatric populations. The chapter presents a case study with differential diagnoses and risk factors for comorbidities. It discusses substance use disorder assessment tools including evidence based treatment options. Treatment options are dependent on the screening, diagnosis and assessment of the substance, psychosocial stressors and dual diagnosed psychiatric disorder. The chapter helps the practitioners to: identify diagnostic criterion for substance use disorders; analyze specific assessments for Alcohol Use Disorder, Benzodiazepine Use Disorder and Opioid Use Disorder; apply assessment tools and evaluate differential diagnoses to identify substance use disorders; evaluate risks and comorbidities of geropsychiatric populations with substance use disorders; and understand treatment options for substance use disorders.