This handbook addresses behavior problems and mental health disorders in persons with intellectual disabilities. It provides an overview of the history of dual diagnosis and related theories, ethics, diagnostic systems, mental health disorders, and challenging behaviors. The handbook examines general clinical issues, such as the effects of cognitive performance on the choice of assessment and treatment methods, service delivery systems, education models, risk factors, functional assessment, and structured interviews. Chapters provide a much-needed reference for practitioners and practitioners in training. The applied focus of the book continues with assessment/diagnosis sections of mental health disorders, and challenging behaviors. In addition, chapters describe treatments for discrete mental health and behavior problems, such as intellectual disabilities, severe psychopathology, autism, ADHD, substance abuse, and aggression. Topics featured in this handbook include: • Genetic disorders and dual diagnosis. • Assessment of anxiety in persons with dual diagnosis. • Aging with intellectual disabilities. • Feeding problems and assessment in individuals with dual diagnosis. • Pica in individuals with intellectual disability. • Treatment of social skills in dual diagnosis. The is an essential reference for researchers, graduate students, clinicians and related therapists and professionals in clinical child and school psychology, child and adolescent psychiatry, social work, developmental psychology, behavioral therapy/rehabilitation, pediatrics, and special education.