Numerous somatic and psychopathological disorders occur parallel with autism spectrum disorders. It is presently being discussed whether other co-occurring psychopathological symptoms should constitute a categorical comorbid disorder on their own right and be diagnosed as such; or whether they should be understood as part of the autistic disorder itself. Based on the situation with attention deficit/hyperactivity disorder (ADHD), the current state of research and our own research results are used as examples to demonstrate which prerequisites must be fulfilled for a comorbid disorder. Furthermore, based on neurobiological findings from the areas of molecular biology, neuropsychology, and imaging we show which requirements emerge for the aetiology, early detection, pharmacological and psychotherapeutic treatment, course of illness, and the categorical classification approach.