To date, research exploring experiences of diagnosing autism spectrum disorder has largely focused on parental perspectives. In order to obtain a more complete account of the autism spectrum disorder diagnostic process, it is essential that the views and experiences of professionals are heard. In this study, 116 multidisciplinary professionals involved in diagnosing autism spectrum disorder in the United Kingdom completed an online questionnaire exploring their experiences and opinions of three key areas of service: accessibility, the diagnostic process and post-diagnostic support. Although professionals were largely satisfied with service accessibility, around 40% of services were failing to provide timely assessments. Standardised diagnostic tools were perceived as helpful and were used consistently, but concerns were raised about their validity in detecting atypical autism spectrum disorder presentations (e.g. females). Several challenges regarding giving autism spectrum disorder diagnoses were reported; these included making sure caregivers understood the diagnosis, pitching information at the correct level and managing distress. Furthermore, the practice of ‘upgrading’ to a diagnosis of autism spectrum disorder in uncertain or complex cases was reported by many, albeit infrequently, and reasons for this varied widely. Professionals expressed dissatisfaction with post-diagnostic provision, especially onward and long-term support options. They also felt that service improvements were required across populations and across the three key areas of service.