Autism Spectrum Disorder (ASD), a lifelong neurodevelopmental disorder characterised by qualitative impairments in social communication together with patterns of restricted, repetitive and often stereotyped interests and behaviours, affects approximately 1% of the population. The revised DSM 5 criteria, published in 2013, now use ASD as a single diagnostic category. Clinical presentations will vary between individuals and across chronological and developmental age and so possible signs of ASD must be interpreted with reference to the normal range of skills expected for a given age and in the context of an individual’s broader developmental profile. Where there are concerns about possible ASD, a referral should be made for an ASD diagnostic assessment. In addition to determining whether an individual reaches the required criteria for an ASD diagnosis and considering relevant differential diagnoses, a multidisciplinary ASD diagnostic assessment should provide an individual skills-andneeds- based profile and identify co-morbidity. The multidisciplinary team’s diagnostic formulation and individualised recommendations about future support and interventions (informed by the skillsand- needs profile), should be shared with the family/carers, and where appropriate the individual undergoing assessment. With consent, the diagnostic report can also be shared with relevant education, social care and health care services.