The origin of ADHD is multifactorial and both the aetiology and pathophysiology of ADHD are as yet incompletely understood. The monoamine deficit hypothesis of ADHD postulates a dysbalance in the interaction of the neurotransmitters dopamine, noradrenaline and serotonin. Pathophysiological mechanisms involved in ADHD include alterations in fronto-striatal circuits. The currently proposed animal models of ADHD are heterogeneous with regard to their pathophysiological alterations and their ability to mimic behavioural symptoms and to predict response to medication. Some evidence points to a genetic basis for ADHD which is likely to involve many genes of small individual effects. In summary, specific neurobiological substrates of ADHD are unknown and multiple genetic and environmental factors appear to act together to create a spectrum of neurobiological liability.