In providing an overview of the mental health system, part 1 discusses the shift from large institutions to a community-based system of care, which generally consists of a fragmented network of providers that is organized differently from State to State. Part 1 also explains the relationship between the decrease in State hospital populations and the corresponding increase in people with mental illnesses in the criminal justice system, as well as the diverse funding mechanisms that support mental health services. Part 2 explains the current understanding of mental illnesses as genuine neurobiological diseases of the brain that can be managed at levels of effectiveness comparable, or superior, to the treatment of physical illnesses. The four main types of symptoms of mental illness are also discussed, as is the difficulty in identifying some defendants with mental illnesses whose symptoms may not be immediately obvious. Part 3 addresses how mental illnesses are diagnosed. It also considers the typical diagnoses of people with mental illnesses who come before the court and their frequent co-occurrence with substance-use disorders. Part 4 presents guiding principles for quality care, such as client centeredness, responsiveness to individual needs, recovery, cultural competency, and evidence-based practices. Various evidence-based treatment practices are explained. Part 5 discusses treatment and support needs for individuals with mental illnesses. It reviews the range of coordinated, high-quality, community-based services needed to address the extensive mental health and social welfare needs of individuals with mental illnesses. Part 6 focuses on how to coordinate treatment and court-based services. 84 notes and appended common medications used in treating mental illnesses