Attention-deficit hyperactivity disorder (ADHD) is a commonly diagnosed childhood psychiatric disorder. Debate over its diagnostic validity, aetiology, presentation and treatment has extended from the clinical to the public domain. As children with ADHD diagnoses reach adulthood there is increasing interest in ‘adult ADHD’. Cohorts followed up show poorer outcomes as adults than do controls. Self-referred adults, sometimes relatives of children with ADHD, are also of interest regarding adult ADHD. Innovative work is being done examining issues of aetiology, treatment, outcomes and comorbidity in these groups, but heterogeneity among those diagnosed with ADHD and changes in classification systems and diagnostic criteria over time complicate comparison of research findings. The diagnostic validity of adult ADHD remains uncertain and needs further study.