Behavioral health and substance use disorders affect approximately 20 percent of the U.S. population. Of those with a substance use disorder, approximately 60 percent also have a mental health disorder. Together, these disorders account for a substantial burden of disability, have been associated with an increased risk of morbidity and mortality from other chronic illnesses, and can be risk factors for incarceration, homelessness, and death by suicide. In addition, they can compromise a person's ability to seek out and afford health care and adhere to treatment recommendations. 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.