Autistic Disorder was described by Leo Kanner in 1943. Since that time not only the name of this disorder (initially early infantile autism) has changed but also it's relation to other disorders. DSM-IV includes autism together with Rett's Disorder, Childhood Disintegrative Disorder, Asperger's Disorder and Pervasive Developmental Disorder Not Otherwise Specified into one category: Pervasive Developmental Disorders. The definition and contents of Pervasive Developmental Disorders raise many controversies. Differentiation between particular disorders within this category is also difficult. This paper discusses some of these problems.