This chapter reviews the limited data that exist in the field of autism, starting with what is known in children. Given that both attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) show a strong tendency to persist from childhood into adult life, it is important for understanding the overlap of the two categories in adulthood to have a good knowledge base of their childhood comorbidity. The chapter discusses some of the diagnostic boundaries and problems in the field. It then considers the issue of whether features of autism occur in ADHD. Sometimes ASD is the diagnostic category that applies early in life, and criteria for ADHD are met only later in childhood. In other individuals, both disorders coexist throughout childhood and adolescence. A large scale study of adult psychiatric patients with attention deficit and social interaction problems found a considerable degree of overlap between ASD and ADHD.