A psychiatric disorder refers to a significant dysfunction in an individual’s cognitions, emotions or behaviours that reflects a disturbance in the psychological, biological or developmental processes underlying mental functioning. Comorbidity refers to two or more forms of psychopathology within the same person. For some, a second diagnosis requires symptom presentation without assumptions of causality (i.e. disorder one causes disorder two). For others, a second diagnosis requires the dysfunction to be significant enough to require treatment in addition to the primary condition.There are several difficulties in assessing comorbidity in people with ASD:There is diagnostic overshadowing.There is often a different symptom presentation in children with ASD than in children without ASD.There are no clear systematic methods of identifying and treating comorbid psychiatric disorders across the autism spectrum.Many symptoms of psychopathology overlap with symptoms of ASD.ASD is very heterogeneous making complete distinguishability of ASD and other disorders difficult.The presence of intellectual disability complicates diagnosing other disorders.Diagnostic overshadowing refers to not recognizing a second disorder because the problems are attributed to the first disorder. Intellectual disability is common in ASD. There are several benefits of having accurate and early recognition of comorbidity:Early treatment has substantial effects on cognitive ability (1), adaptive behaviour (2) and psychopathology (3).Knowledge on comorbid disorders can inform treatment planning.Knowledge on comorbid disorders can focus researchers on developing specific diagnostic tools (1), treatments (2) and psychopharmacology (3). Comorbid disorders may cause impairment in learning, which makes the treatment of ASD more problematic. There are few standardized assessment tools to assess psychiatric symptoms among individuals with ASD. Tools developed to assess...