Pervasive developmental disorders (PDD) or autistic spectrum disorders (ASD) encompass a heterogeneous group of individuals with early childhood onset of deficits in social interaction and language development, a restricted repertoire of interests and activities, as well as a wide range of cognitive difficulties. The DSM-IV divides the PDDs into five behaviorally defined categories: autistic disorders, Asperger syndrome, Rett Syndrome, disintegrative disorder, and pervasive disorder not otherwise specified (NOS). In a significant minority (20–40 %) of these children, parents report observing deterioration in the child’s language skills, accompanied with deterioration of nonverbal communication and play skills. Behavior often worsens and cognitive decline may also be observed. At least some of these children are entirely healthy prior to the onset of the disorder. In particular Disintegrative Disorder refers to a subgroup of children on the PDD spectrum who were developing entirely normally, including speaking in sentences, in whom deterioration occurred after 2 years of age. In some of these children, onset of deterioration may be as late as mid-childhood attesting to the significant heterogeneity among individuals diagnosed with Autism. Furthermore, while much research into the biological factors contributing to the development of disorders on the spectrum is underway, the constellation of factors necessary for the syndrome to manifest remains far from clear.