Comorbidity, the co-occurrence of two or more disorders in the same person, has been a topic receiving considerable attention in the child psychopathology literature overall. Despite many publications in the ADHD, depression and other child literatures, autism spectrum disorder has not received such scrutiny. The purpose of this review will be to discuss the available evidence. We address specific variables in diagnosis and classification of comorbid symptoms, and propose potential avenues for research and practice with respect to differential diagnosis. A brief discussion of the implications for treatment is also provided.