Book excerpt: This textbook provides a state of the art of the knowledge on the prevalence, risk and etiological factors, clinical features, assessment procedures and tools, diagnostic criteria, treatment, and prognosis of the psychiatric disorders encountered in people with intellectual disability (ID) and low-functioning autism spectrum disorder (ASD). ID and ASD represent two meta-syndromic groups of several different conditions, each with particular cognitive and communication features. People with ID/ASD display an increased prevalence of a variety of psychiatric disorders, including psychotic disorders, mood disorders, anxiety and stress-related disorders, somatoform disorders, and obsessive-compulsive disorder, as well behavioral syndromes, personality disorders, and disorders due to psychoactive substance use. This book will enable readers to understand the specificities of psychiatric disorders in the context of ID/ASD. It explains clearly how diagnostic criteria and assessment procedures for psychiatric disorders that were created for the general population have to be modified for use with ID/ASD. Above all, it will enable clinicians to overcome difficulties in diagnosis and to deliver more effective care that meets the particular needs of patients with ID/ASD.