Parent-teacher agreement on Attention Deficit/Hyperactivity Disorder (ADHD) symptom ratings range from low to moderate. Most studies evaluating parent-teacher agreement have not assessed measurement invariance across raters. Hence, it is unclear whether discordance across raters is due to differing ADHD constructs across raters or other factors (e.g., subjective differences across raters). Additionally, the effect of development on parent-teacher agreement is relatively unknown. To address these limitations, the present study utilized parent and teacher ADHD ratings from a large (N=6,659), developmentally diverse (ages 4–17) sample. Exploratory structural equation modeling using half the sample and then Confirmatory Factor Analysis (CFA) on the second half of the sample identified a two-factor structure for the 18 ADHD symptoms. CFA invariance analyses demonstrated that the two factor symptom structure was similar across raters and age groups. Also, parents reported greater severity of ADHD symptoms than teachers and both parents and teachers report higher levels of HI in younger children than older children and consistent levels of inattention across development. Finally, correlations between parent-teacher ratings of like-factor ratings were weak for Inattention (IA) and moderate-strong for Hyperactivity/Impulsivity (HI) and that the magnitude of parent-teacher agreement did not vary across development. In conclusion, while parent and teacher ratings of ADHD behaviors are only weakly to moderately correlated, each reporter provides unique and valid clinical information as it relates to ADHD symptom presentation.