Current best practice guidelines advocate a multidisciplinary approach to the diagnosis and management of attention-deficit hyperactivity disorder (ADHD). However, no research has yet examined whether parents and professionals share common beliefs about ADHD. This study examined the implicit beliefs about the characteristics and causes of ADHD held by parents and professionals in the psychological, educational, medical, and allied health fields. A total of 670 participants rated 46 characteristics and 29 causal explanations for ADHD. Principal component analyses identified five characteristic factors and six causal factors. There was a high degree of concordance between the groups in their beliefs about the characteristics and causes of ADHD. However, several important differences were also found. The implications of the overlap and differences between professional and nonprofessional groups, and the scientific literature are considered.