Autism is a disorder of social communication, originally described as a triad of impaired social interaction, communication, and imagination, associated with a rigid, repetitive pattern of behaviour. More recently the core deficit has been described as a lack of social instinct. The term autism spectrum disorder (ASD) includes autism, atypical autism and Asperger's syndrome. According to NICE, diagnostic assessment should start within three months of referral. The decision to refer should be made on the basis of signs or symptoms. Information from all sources (i.e. medical history, parental concerns, clinical judgement) should be used to diagnose ASD based on ICD-10 or DSM-IV criteria. Clinicians should not rely on any one autism-specific diagnostic tool alone to diagnose ASD. Diagnosis is based primarily on the recognition and interpretation of behavioural symptoms. CHAT or the modified version (M-CHAT) are simple checklists that can be used for the assessment of young children in primary care when ASD is suspected. They can identify clinical features indicative of increased risk but should not be used to rule out ASD.