This thesis uses a multi-disciplinary approach to study cognitive-neurophysiological processes underlying attention-deficit/hyperactivity disorder (ADHD), and the underlying risk pathways from preterm birth to ADHD. In the first part of the thesis we use a measure of peripheral arousal (skin conductance) to better understand arousal dysregulation in ADHD and how it relates to cognitive performance. We show, using a large ADHD and control sibling sample, that ADHD is associated with peripheral hypo-arousal, and that a familial aetiology underlies the relationship between hypo-arousal and fluctuating reaction times, and between hypoarousal and ADHD. Our findings further suggest that peripheral hypo-arousal is an enduring deficit in ADHD, as it is observed in both ADHD remitters and ADHD persisters in our followup investigation. The second part of the thesis focuses on preterm birth as a risk factor for ADHD: we compare data we obtain from a new sample of preterm-born adolescents and their siblings to data from ADHD and control sibling pairs. First, we find that preterm-born individuals show several of the same cognitive-neurophysiological impairments as individuals with ADHD, but they also show further, additional impairments. Second, our results indicate that cognitive-neurophysiological impairments in the preterm group differentiate into those that are in line with a causal effect of preterm birth, and those that are not. Third, our findings further suggest that the association between ADHD symptoms and specific cognitive impairments is largely due to familial influences among term-born individuals, but largely due to non-shared effects (including preterm birth as an environmental insult) among pretermborn individuals. Overall, by using a combination of cognitive, neurophysiological, developmental and sibling-comparison designs, our findings provide new insight into arousal dysregulation in individuals with ADHD, and inform on cognitive-neurophysiological and aetiological processes that may underlie the association between preterm birth and ADHD.