Objective: To investigate time perception in Attention-Deficit/Hyperactivity Disorder (ADHD) with and without comorbid reading difficulties (RD) in child and adolescent participants. Method: In study 1, 50 children with ADHD (31 ADHD, 19 ADHD+RD) and age-matched healthy controls (n=50), completed three psychophysical tasks: duration discrimination (target duration of 400 ms versus a foil duration), frequency discrimination (a control condition to evaluate general perceptual ability), and a duration estimation task using the method of reproduction for intervals of 400 ms, 2000 ms, and 6000 ms. Study 2 used the same tasks with an adolescent sample (35 ADHD, 24 ADHD+RD, 39 controls). Results: In both studies, children and adolescents with ADHD and ADHD+RD displayed some impairments in duration discrimination and the precision with which they reproduced the intervals on the estimation task, particularly the shorter 400 ms interval. The most severe impairments tended to occur in the comorbid ADHD+RD group. No impairments were found on the frequency discrimination task. ADHD participants also displayed significant intraindividual variability in their performance on the estimation task. Finally, working memory, estimated full-scale IQ, and teacher report of hyperactivity/impulsivity were found to differentially predict performance on the time perception measures in the adolescent clinical sample. Conclusions: Deficits in duration discrimination, duration estimation, and intra-individual performance variability may have cascaded effects on the temporal organization of behaviour in children and adolescents with ADHD and ADHD+RD