In an ever-changing world, a vital skill for survival is the preparation of possible actions, the monitoring of a selected action and the termination of an inappropriate action. Therefore, preparation, selection, monitoring and inhibition are all core aspects of executive function; they are also some of the key deficits in attention deficit hyperactivity disorder (ADHD). Psychometric measurements of executive function tend to capture only the behaviours, not the core symptoms of these deficits. Event-based electrophysiological brain measures can reveal the neural mechanisms of executive function and can distinguish subtypes of ADHD. The combination of the neurological and behavioural markers of executive dysfunction in ADHD can give an endophenotype diagnosis of condition and inform more personalised treatment plans.