Attention-Deficit/Hyperactivity Disorder (ADHD) is a common childhood disorder, yet relatively little research has assessed teachers' knowledge of this disorder or how teacher characteristics affect such knowledge. There also is a dearth of research comparing in-service and preservice teachers' knowledge about ADHD. The aims of this study were (a) to investigate the relationships between various teacher characteristics and teachers' knowledge about ADHD, and (b) to compare perceived and actual ADHD knowledge across in-service and preservice primary-school teachers. Participants included 120 primary-school teachers and 45 final-year education undergraduates who were asked to complete a questionnaire. Two of the five hypotheses were supported: (a) that knowledge would be significantly correlated with experience in teaching a child with ADHD and (b) that in-service teachers would obtain higher scores than preservice teachers on the actual knowledge (about ADHD) questionnaire. Implications stemming from this research include ensuring that teachers receive more training about ADHD and greater exposure to students with ADHD in the classroom. Overall, this study highlighted that deficits in teachers' knowledge about ADHD are common for both in-service and preservice teachers. (Contains 2 tables.)