Hyperactivity is still sometimes portrayed as an arbitrary, unreliable and even invalid diagnostic entity that adds nothing to, or even obsucures, our understanding of disruptive behaviour disorders in children. The evidence, however, supports a different view. The diagnosis, based as it has been largely on clinical observation and consensus, has proven useful for clinical practice and science. The studies performed in the past decade indicate a prevalence of between 5 and 10%, with the higher rate mostly found among primary school-age boys. Lower figures have been reported for girls, preschoolers and adolescents. The disorder shows a high degree of persistence over time, and a substantial comorbidity with learning problems, oppositional/conduct disorders, depression and anxiety disorders. This second edition not only presents thoroughly updated reviews of the topics covered in the first edition, but also ventures into new fields such as genetics, the effects of early institutional rearing and the adult form of the disorder. As was the case first time round, a group of internationally renowned experts and leading researchers in the fields of psychology, psychiatry and brain sciences have been brought together to present a multifaceted but interlocking account of the condition. Despite recent attempts at harmonization, differences in diagnostic practice continue to influence the prevalence of the disorder, resulting in 10- or even 100-fold differences being quoted between the USA and the UK, for example. Including or excluding coexisting disorders and preference for a categorical over a quantitative approach are some of the reasons for this.