Objective: The study examined executive function deficits (EFD) in school-age children (7 to 14 years) with ADHD. Method: A clinical sample of children diagnosed with ADHD (n = 49) was compared to a population sample (n = 196) on eight executive function (EF) measures. Then, the prevalence of EFD in clinical and non-clinical children was examined at the individual level according to three methods previously applied to define EFD, and a fourth method was included to control for the effect of age on performance. Results: Children with ADHD were significantly more impaired on measures of EF than children without ADHD at the group level. However, only about 50% of children with ADHD were found to have EFD at the individual level, and results appeared relatively robust across methods applied to define EFD. Conclusion: As a group, children with ADHD displayed more problems on neuropsychological measures of EF than non-clinical children; at the individual level, there appeared to be heterogeneity in EF impairment. (Contains 4 tables.)