Factor analytic studies have been conducted to examine the inter-relationships and degree of overlap among symptoms in Autism Spectrum Disorder (ASD). This paper reviewed 36 factor analytic studies that have examined ASD symptoms, using 13 different instruments. Studies were grouped into three categories: Studies with all DSM-IV symptoms, studies with a subset of DSM-IV symptoms, and studies with symptoms that were not specifically based on the DSM-IV. There was consistent support for a common social/communication domain that is distinct from a restricted and repetitive behaviours and interests domain. Implications for symptom conceptualization and diagnosis in ASD are discussed.