This chapter discusses what ADHD is, including an overview of the features of ADHD, features of the inattentive type of ADHD (poor concentration, task impersistence, disorganization, forgetfulness), features of the combined type of ADHD (impulsivity, overactivity, noisiness), and features that may be present in either type of ADHD (performance inconsistency, low self-esteem, poor working memory, poor incentival motivation, social clumsiness, learning difficulties, inflexibility, insatiability, defiant behaviour, sleep problems), as well as associated conditions, prevalence, and a historical background.