Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental condition affecting 5.9% of young people. Late adolescence can be a particularly challenging period for young people with ADHD, with major life transitions, new demands and increased expectations. This vulnerable phase also coincides with the transition from child and adolescent mental health care to adult ADHD services, where new UK data show that most young people with ADHD do not successfully transfer to adult services. Therefore, many young people with ADHD do not receive appropriate interventions at a time when they may need them most. Opportunities for intervention are currently not fully realised due to both the young people's disengagement from clinical services and our limited understanding of real-world targets for more holistic interventions. The current study seeks to address these needs using remote (not in-person) measurement technology (RMT). The MRC-funded project, ART-transition, will use the ADHD Remote Technology ('ART') assessment and monitoring assessments with young people with a diagnosis of ADHD aged 16-17 and the RADAR-base mobile-health platform to which it is linked. ART consists of active (e.g. questionnaires) and passive (e.g. sleep) smartphone app monitoring. In the study, the investigators will address three questions on the transition to adulthood for individuals with ADHD: what changes take place, what predicts them, and how can the investigators prevent negative outcomes and support healthy lifestyles? The investigators will remotely monitor 250 young people with ADHD over two years. The investigators will then co-design, with young people with ADHD, a prototype for a new ADHD-transition smartphone app. Our approach focuses on giving young people with ADHD greater autonomy in how they manage their ADHD, in collaboration with their clinician, and places the emphasis on modifiable environmental factors and the prevention of negative outcomes.