Autism Spectrum Disorders (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are both highly heritable neuropsychiatric disorders. Together they account for over 50% of the yearly new referrals in child and adolescent psychiatry. There is substantial comorbidity between both disorders, which has received little research attention due to a comorbid diagnosis restriction in the DSM-IV. With the removal of this restriction in the upcoming DSM-5, it is vital to gain more insight in the causes and consequences of a comorbid diagnosis. In this symposium, a series of inventive studies will be presented illustrating to what degree ASD and ADHD (non-)overlap regarding diagnostic information and cognitive profiles, to what extent the low end of the ASD/ADHD spectrum indeed represent superior functioning, if simplex and multiplex families show differential patterns of cognitive problems and to what degree parental ASD/ADHD symptoms influence the family environment in families with ASD/ADHD affected offspring. Strong emphasis will be given on the clinical implications of these findings.