A diagnosis is the formal identification of autism, usually by a health professional such as a paediatrician or psychiatrist. Having a diagnosis not only helps people with autism and their families to understand why they may experience certain difficulties, but also enables them to access services and support.

Due to recent and upcoming changes to the main diagnostic manuals, "autism spectrum disorder" (ASD) is now likely to become the most commonly given diagnostic term. However, some clinicians may use additional terms to help to describe the particular autism profile presented by an individual. These may include Aspergers Syndrome, classic autism, high functioning autism and pervasive developmental disorder (PDD). Some autistic people also have learning disabilities, mental health issues or other conditions.