© Cambridge University Press 2007 and Cambridge University Press, 2009. Summary of findings Introduction Psychosis has carried different meanings since its introduction more than 150 years ago (Beer, 1996). Others have described the social and intellectual contexts that have shaped the concept of psychosis at different times and places (Berrios, 1987; Beer, 1995). Modern classification systems incorporate psychosis in various disorders as a serious disturbance in “reality testing” expressed as hallucinations, delusions, thought disturbance, disorganized behavior, or catatonia. Recent advances in neuroscience hold the promise of elucidating the brain mechanisms of psychosis and finding improved antipsychotic treatments. Fujii & Ahmed (2004) have recently proposed conceptualizing psychosis as a neurobiological syndrome with its own pathophysiology and treatment algorithm regardless of etiological factors and underlying diagnoses. This view has heuristic value for refining current classification systems, focusing research, and tailoring treatments on an individual basis. In this chapter, we report on the characteristics of psychosis superimposed on autism. First, some definitions of autism are presented. In DSM-IV classification, autism covers the group of the Pervasive Developmental Disorders (PDDs) (American Psychiatric Association, 1994), i.e., Autistic Disorder, Asperger Disorder, Childhood Disintegrative Disorder, Rett's Disorder, and Pervasive Developmental Disorder Not Otherwise Specified (PDD NOS). The PDDs are behavioral syndromes with a broad range of severity and characterized by lifelong impaired communication, impaired social interactions, and repetitive interests and behavior (Wing & Attwood, 1987; Rapin, 1997). DSM-IV diagnostic criteria of AD, AsD, and CDD, are shown in Table 12.1.