The purpose of this study was to examine the relationships between ADHD symptoms and impairment within a sample that includes children in early childhood (n = 250), middle childhood (n = 269) and adolescence (i.e., high school; n = 269). Nested multivariate regression analyses were used to examine the extent to which each ADHD symptom dimension (inattention and hyperactivity/impulsivity) is most associated with impairment in academic, social, and behavioral functioning within and across three developmental levels. Results indicated for academic impairment, inattention is more predictive than hyperactivity/impulsivity at all ages. For social functioning, both with peers and teachers, hyperactivity/impulsivity is more predictive than inattention but only for early childhood students; at the high school level, inattention is a significant predictor of social impairment. For behavioral functioning in the classroom, the pattern is mixed across dimensions and ages with HI decreasing in predictive utility across the three age groups and IA increasing in predictive utility. Forward stepwise regression was used to determine which of the 18 ADHD symptom items are most associated with impairment within and across developmental levels. Findings indicate that the symptoms that were most predictive of impairment varied by age group and by domain of impairment. Implications for assessment are discussed.