Autism Spectrum Disorder (ASD) is characterized by difficulties in social communication, and repetitive and restricted patterns of behaviors and interests. In the first official publication of a collection of case studies on autism, Leo Kanner (1943) documented the symptoms of autism as autistic disturbances of affective contact. Since then, the conceptualization of autism as a disorder has gone through many levels of revisions. Decades of research, scientific publications, changes in the diagnostic and statistical manual of mental disorders, and experiences of clinicians and autistic individuals have all contributed to improve identification and treatment of ASD. In this chapter, we attempt to examine the history and evolution of the definition and characterization of autism over the past several decades and the impact of it on how the disorder is perceived by autistic individuals, families, and professionals.