The foundation of child and adolescent psychiatry is developmental psychopathology. Not only in the physical realm but also in the mental sphere, there is a continuous age- and development-dependent change in manifestations. Earlier attempts to retroactively derive the psychological symptoms of children and adolescents from the clinical pictures of adults stand in contrast to their entelechy. However, a psychopathology that would be valid for all developmental and age stages is also a fiction. Even newborns and infants are very competent beings, and toddlers, school-aged children, adolescents, and young adults make such rapid developmental progress, sometimes almost abruptly, that even with subtle examination techniques, a reliable nosography of mental disorders is not always possible.