To explore data, policies, practices, and systems that affect the diagnosis and provision of care for mental health and substance use disorders, the Health and Medicine Division of the National Academies of Sciences, Engineering, and Medicine created the Forum on Mental Health and Substance Use Disorders. The forum activities are expected to advance the discussion and generate potential ideas on ways to address many of the most persistent problems in delivering mental health and substance use services. The inaugural workshop, held October 15-16, 2019, in Washington, DC, explored the key policy challenges that impede efforts to improve care for those individuals with mental health and substance use disorders. This publication summarizes the presentations and discussion of the workshop.