This guideline covers the recognition, referral and diagnosis of autism in children and young people from birth up to 19 years. The term ‘autism’ describes qualitative differences and impairments in reciprocal social interaction and social communication, combined with restricted interests and rigid and repetitive behaviours. Autism spectrum disorders are diagnosed in children, young people and adults if these behaviours meet the criteria defined in the International Statistical Classification of Diseases and Related Health Problems (ICD-10) and the Diagnostic and Statistical Manual of Mental Disorders DSM-IV Fourth Edition (Text Revision) (DSM IV-TR) and have a significant impact on function. The over-arching category term used in ICD-10 and DSM-IV-TR is pervasive developmental disorder (PDD), a term now used synonymously with autism spectrum disorder (excluding Rett syndrome): it is a behaviourally defined group of disorders, which is heterogeneous in both cause and manifestation. The guideline development group recognised that individuals and groups prefer a variety of terms, including autism spectrum disorder, autistic spectrum condition, autistic spectrum difference and neuro-diversity. For clarity and consistency, in this guideline the term ‘autism’ is used throughout in keeping with the use of ‘autism’ in recent Department of Health, National Audit Office and Public Accounts Committee documents. However, in this guideline ‘autism’ refers to ‘autism spectrum disorder’.