The goals of this research are to assess the influence of timing mechanisms on executive function for adults diagnosed with predominantly inattentive or combined subtypes of ADHD. Specifically, the experiments outlined include a measure of temporal acuity and task switching paradigms in which timing, predictability and response output are manipulated. These factors are integral to the temporal organization of behavior and attention. Participants will include those for whom childhood and adult diagnoses are consistent (predominantly inattentive or combined) in order to make inferences about developmental trajectories of specific brain areas known to be critical for timing and prediction/error correction. Ultimately, the cognitive tasks outlined would be transitioned to functional imaging protocols to correlate behavioral data with patterns of brain activation specifically related to aspects of timing. Data from these studies could inform current models of ADHD and underlying circuitry, an ultimately contribute to more effective assessment and behavioral/pharmacologic intervention strategies.