Attention deficit hyperactivity disorder (ADHD) is a controversial diagnosis. Ongoing debates on impact of neurobiological differences and sociocultural demands on behavioural responses influence all levels of ADHD healthcare. A theoretical interpretation of impairments as deficits in executive functioning and self-regulation is the foundation for this debate. This thesis questions these interpretations as the basis for design of research, treatment approaches and outcomes. A scoping review of 278 documents explores this theoretical influence on adult treatment and presents an alternative aetiology of ADHD based on Self-Determination Theory (SDT). New research presents a grounded theory study of 11 adult participants exploring the lived experience of ADHD variable impairment. Results produced the ADHD Creative Awareness Theory (CAT) – a new theory of ADHD consciousness and environmental engagement recognising a natural ADHD self-regulation which includes treatment recommendations. A randomised feasibility study of the ADAPT Framework - an SDT-based psychosocial intervention for ADHD adults was designed to test these recommendations. The study examines intervention feasibility, acceptability, and adherence and suitability of outcome measures. Twenty participants were randomised to an 11-session online intervention (n=11) or waitlist control (n=9) condition. Following waitlist completion, control participants were offered the intervention. Primary outcomes were acceptability and feasibility evaluated quantitatively through participation rates, attrition rates, and missing values from pre- and post-condition measures. Secondary outcomes included assessing suitability of chosen measures and analysis of intervention effectiveness. All participants contributed to a qualitative interview question regarding intervention significance. The study found the ADAPT Framework is a feasible and acceptable non-deficit intervention for adults with ADHD. Further, the SDT-based perspective is an empowering approach to support adult ADHD self-development, wellbeing and functioning. Future RCTs would benefit from self-development measures of change and long-term follow-ups to assess intervention maintenance. Consideration of in-person delivery, context specific group applications and alternative client groups should be included.