Living with schizophrenia, bipolar disorder, or other SMI while navigating addiction is not “two problems”—it’s one complex reality. Severe Mental Illness and Substance Use focuses on fully integrated care that treats psychosis/mood instability and substance use as intertwined drivers of risk, disability, and relapse. We begin with why co-occurrence is common: shared neurobiology (reward, salience, stress), social determinants (housing, trauma, poverty), and health-system fragmentation. The session maps an evidence-based path: one team, one plan, one chart. Screening for use patterns and withdrawal sits alongside assessment of positive, negative, cognitive, and mood symptoms. Medication choices balance efficacy with safety: antipsychotics and mood stabilizers are coordinated with MOUD/anti-craving agents; interactions, QTc, and metabolic risk are tracked systematically. Psychosocial care is staged—engagement and harm reduction, skills and structure, then recovery roles—using contingency management, CBT/ACT elements, and family partnership. We address high-risk interfaces: intoxication-triggered aggression, suicidality, medical comorbidity, and homelessness; and we show how to build hospital-to-community bridges that actually hold (peer outreach, pharmacy synchronization, rapid tele-follow-ups). Measurement focuses on function, sleep, cravings, and safety—outcomes that matter to patients and families. Equity is built in: language access, simplified regimens, and practical supports (transport, food, ID restoration). Finally, we describe sustainable operations: co-located clinics or virtual integration, shared protocols, and dashboards that drive weekly course-correction. With consistent structure, Severe Mental Illness and Substance Use, the visibility from a SMI and SUD conference, and field standards for integrated dual diagnosis care make recovery more likely—and more durable.