Objective: To identify what primary and secondary school teachers know about ADHD in children and, furthermore, to identify which factors predict their knowledge. Method: A 29-item questionnaire about ADHD was distributed to a random, nationwide, and representative sample of Danish primary and secondary school teachers. Data were analyzed descriptively and by hierarchical regression analysis. Results: A total of 528 teachers were included. Most teachers identified the symptoms of ADHD (79%-96%) and effective classroom intervention strategies (75%-98%). However, knowledge about other characteristics, etiology, prognosis, and treatment was inconsistent, for example, only 56% and 17% correctly rejected diet as a cause and effective treatment for ADHD. Among the strongest predictors for correct knowledge was having been provided postgraduate education about ADHD. Conclusion: Teachers require knowledge about ADHD to successfully include and manage children with ADHD and, additionally, to ensure positive working environments for teachers and support constructive school–home working collaborations.