Providing evidence about how to improve diagnostic services for adults with Autism Spectrum Disorder (ASD) is a research priority. This study (within the Autistica funded ASD lifecourse and ageing research programme) will identify efficient and effective methods and measures for the diagnosis of ASD in adults across the age and ability range, in the UK National Health Service (NHS). Together with collaborators we will:1. Survey adults with an ASD diagnosis, those seeking a diagnosis and relatives about their experience of NHS services. We will also survey UK professionals who diagnose adults with ASD, about current NHS ASD diagnostic care pathways.2. Using information from surveys, and discussion groups, investigate what people consider is the best way to assess and diagnose ASD across NHS services.3. Update the National Institute for Health and Care Excellence review of measures to ensure we are aware of the latest findings about measures used in diagnosis.4. Through our links with local, national and international clinical services, identify which combinations of measures are most effective for diagnosing ASD in adults across the age and ability range.5. Revise and pilot an existing measure of social communication and rigidity behaviours used extensively with adults. We will evaluate the utility of this diagnostic interview that can be used when little/no early developmental history is available.The findings about efficient and effective ASD diagnostic services, and measures will be made available to the ASD community and health professionals. This will provide NHS policymakers and professionals with evidence based information about: the best way to measure an individual’s profile of strengths and difficulties; how to provide efficient and effective services; and what are the types of services most likely to benefit people with ASD and their relatives.