This special issue represents an attempt to answer fundamental brain and behaviour issues in attention-deficit hyperactivity disorder (ADHD). The European network on hyperkinetic disorders (Eunethydis) is trying to develop a novel, testable theory of ADHD, giving an account of its causes, its development from brain dysfunctions to behavioural symptoms and co-morbidity and explaining why no current therapy produces long-lasting improvements. The combined insights of the articles presented here suggest that there is no brain damage in ADHD, but hypo-efficient dopamine systems which give rise to neurochemical imbalances. These cause behavioural problems: deficits in sustained attention, overactivity and impulsiveness. Impulsiveness is increasingly being seen as a key characteristic of the disorder. None of these symptoms are necessarily primary, but may be secondary to an underlying deficit in reinforcement processes seen particularly in a greater than normal sensitivity to variations in the timing of stimulus presentation. Other symptoms can also be seen: altered effects of reinforcers, increased behavioural variance and motor co-ordination problems. Medication produces temporary, plastic changes in cellular components like receptors and transduction mechanisms normalising dopamine functions and behaviour. reserved.