This study aims at better understanding the knowledge and beliefs of professional workers in school, health and social service settings in Quebec regarding ADHD. The authors examine the important discrepancies identified by Cohen (1999) between identified standard practices in treating ADHD patients and practices used in Quebec. This situation could be linked to insufficient knowledge of workers or certain of their beliefs that oppose these practices and their reluctance to implement them in their environment. Two measurement scales were utilised : the Attention Deficit Hyperactivity Disorder Orientation Scale (ADHDOS, Couture, 2002) and the Survey of ADHD of Jerome and al. (1994). Results show among other things, that knowledge and beliefs vary according to professionals'background and training.