‘Puzzle pieces falling into place’: experiences with the diagnostic process of developmental disorders or neuropsychological sequelae of an acquired brain injury in adulthood Adults with a (suspected) developmental disorder or neuropsychological symptoms after an acquired brain injury often experience long-term difficulties in various areas of life. Partly due to the less visible symptoms, these diagnoses can stay unnoticed for a long time. This is a problem which developmental disorders and acquired brain injuries have in common. This delay leads to broad psychosocial problems and/or a vulnerable position in society. This study maps the experiences of adults (18 to 65 years) with regard to the diagnosis of a developmental disorder or an acquired brain injury in Flanders (Belgium): 26 adults with one or more recent diagnoses of autism, ADHD, intellectual disability or neuropsychological symptoms after an acquired brain injury were interviewed. Common experiences were searched through an interpretative phenomenological analysis using the software program Nvivo™. The challenges of symptoms prior to receiving a diagnosis are often great. Adults have been looking for answers for a long time. The accessibility of diagnostic services in Flanders is experienced as limited. Primary and secondary care providers play a crucial role in timely referral, but participants experience a shortage of knowledge and expertise among them. For many participants, receiving a diagnosis is the first step in accepting further treatment, although they report a substantial need for a more seamless transition from diagnosis to guidance and advice.