Pervasive Developmental Disorder (PDD) is a group of five disorders characterized by delays in the development of multiple basic functions, including socialization and communication under the ICD-10. The word "pervasive" is used to distinguish this group from "specific developmental disorders", which affect only one function.
According to the Center for Children, "All types of PDD are neurological disorders that are usually evident by age 3. In general, children who have a type of PDD have difficulty in talking, playing with other children, and relating to others, including their family ... the definition set forth in the DSM-IV (American Psychiatric Association, 1994), Pervasive Developmental Disorders are characterized by severe and pervasive impairment in several areas of development - social interaction skills; communication skills; or the presence of stereotyped behavior, interests, and activities."
The five disorders under PDD are:
- Autism: It is a developmental condition that results from a disorder of the human central nervous system. It is diagnosed using specific criteria for impairments to social interaction, communication, interests, imagination and activities. 
- Asperger syndrome: This Pervasive Developmental Disorder is related to autism. Some of its characteristics are: specific and passionate interests, repetitive routines or rituals, large vocabulary, unusual tone or pitch of voice, tendency towards literal thinking, difficulty understanding social norms, limited ability to interact with peers, problems with non-verbal communication, including the restricted use of gestures, unusual facial expressions, and sometimes clumsiness.
- Childhood Disintegrative Disorder: a rare condition characterized by late onset of developmental delays in language, social development, and motor skills. Children with childhood disintegrative disorder develop typically until they reach 3 or 4 years of age. There are some cases in which the disorder strikes at an older age, as late as 10 years. Regression can be very sudden, and the child may even voice concern about what is happening, much to the parent's surprise. Some children describe or appear to be reacting to hallucinations, but the most obvious symptom is that skills apparently attained are lost.
- Rett syndrome: It is classified under the Pervasive Developmental Disorders, but Rett syndrome has identifiable physical differences such as low muscle tone (causing the child to seem "floppy") and stereotyped hand movements such as wringing and waving. It is a neurodevelopmental disorder that causes severe disability in young children, mostly girls. After a period of relatively normal development, children with Rett syndrome lose skills such as language, play, and hand use.
- Pervasive Developmental Disorder - Not Otherwise Specified: It is a pervasive developmental disorder diagnosis for people with most, but not all, of the characteristics of autism. Individuals with PDD-NOS usually have an easier time adapting to life. Usually, the issues focus more on social interaction.
This classification was rendered obsolete by the DSM-5.
- About Pervasive Developmental Disorders - a page from Yale University (Child Study Center) - 
- Pervasive Developmental Disorders - A publication of the National Dissemination Center for Children with Disabilities
- Indiana Resource center for Autism- website for individuals on the autism spectrum, their families, teachers and supporters.