Child and adolescent psychiatry used to be a part of adult psychiatry. In adult psychiatry, patients with schizophrenia are common, and their problems arise after puberty. However, we cannot overlook their problems in the developmental stage. In child and adolescent psychiatry, the constitution of diseases changes gradually. Diseases developing in adulthood, such as schizophrenia, are rare, and those developing in infants, such as PDD or ADHD, are very common. I divided child and adolescent psychiatric diseases into 3 types（developing in infants, children, and adolescents）, and described them. In child and adolescent psychiatry, multi professional staff, such as doctors, nurses, clinical psychologists, psycho‒social‒workers, and school teachers, need to collaborate with each other for effective treatment.