While evidence-based knowledge is increasing regarding assessment and treatment of comorbidity in autism spectrum disorders (ASD) as it occurs throughout the lifespan of an individual, some significant areas require further study. Comorbidities include symptoms of ADHD, obsessive compulsive disorder, mood disorders, impulsivity, aggression, and self-injury. While comorbidity is often incorrectly ascribed to ASD, and therefore missed (diagnostic overshadowing), such individuals are often subjected to multiple treatment trials and side effects before a response is obtained. This Workshop provides an opportunity to discuss and clarify comorbidity in ASD and congenital syndromes. The presenters also review literature regarding progress made in assessment and treatment of comorbid disorders in the ASD population and in congenital syndromes.