Introduction: Nowadays, many researchers apply autism spectrum disorder (ASD) to describe pervasive developmental disorders. In this model, the differences of previous three common pervasive developmental disorders, including autism disorder, Asperger’s disorder, and pervasive developmental disorder not otherwise specified, point to the various levels and not the type of disorder. According to the importance and poor prognosis as well as the lack of awareness of this disorder among Iranian people, this article discusses the most important causes of ASD. Conclusion: There is no single cause has still been known for ASD. However, ASD may be caused by the interactions of neurobiological and environmental factors with genetic susceptibility.