Studies of glucose metabolism and blood flow have suggested a variety of global and focal brain abnormalities in autism. The accurate diagnosis and careful description of associated features of the subjects are essential criteria for obtaining meaningful functional imaging data. The majority of the functional imaging studies published to date have utilized clinical diagnoses of autism based upon DSM-III-R, DSMIV or ICD-10 criteria. This chapter presents a detailed discussion on functional imaging studies in autistic subjects. It is difficult to scan young children and to obtain appropriate age-matched controls; consequently, the majority of functional imaging studies of autistic subjects have employed adolescents and adults. The chapter discusses the problems that need to be addressed in future studies of autism. It is clear that functional imaging studies in young children, as well as new strategies for the assessment of developmental functional imaging data, are necessary to advance our understanding of autism.