Autism spectrum disorder (ASD) is a range of complex neurodevelopment disorders, characterized by social impairments, communication difficulties, and restricted, repetitive, and stereotyped patterns of behavior.Autistic disorder, sometimes called autism or classical ASD, is the most severe form of ASD, while other conditions along the spectrum include a milder form known as Asperger syndrome, and childhood disintegrative disorder and pervasive developmental disorder not otherwise specified (usually referred to as PDD-NOS).Although ASD varies significantly in character and severity, it occurs in all ethnic and socioeconomic groups and affects every age group. In Canada, the diagnosis of ASD is usually provided by a medical doctors or psychologists. ASD diagnostic assessments typically use both direct observation and developmental interviews to inform the diagnosing clinician's clinical judgement based on ASD criteria from the Diagnostic and Statistical Manual of Mental Disorders.With the May 2013 publication of the DSM-5 diagnostic , all autism disorders were merged into one umbrella diagnosis of ASD.

Over the last five years, scientists have identified a number of rare gene changes, or mutations, associated with autism. Research has identified more than a hundred autism risk genes. In around 15 percent of cases, a specific genetic cause of a person’s autism can be identified. However, most cases involve a complex and variable combination of genetic risk and environmental factors that influence early brain development.

In other words, in the presence of a genetic predisposition to autism, a number of nongenetic, or environmental, influence further increase a child’s risk. The clearest evidence of these environmental risk factors involves events before and during birth. They include advanced parental age at time of conception (both mom and dad), maternal illness during pregnancy, extreme prematurity and very low birth weight and certain difficulties during birth, particularly those involving periods of oxygen deprivation to the baby’s brain. Mothers exposed to high levels of pesticides and air pollution may also be at higher risk of having a child with ASD. It is important to keep in mind that these factors, by themselves, do not cause autism. Rather, in combination with genetic risk factors, they appear to modestly increase risk.