Overwhelming evidence indicates that inattention, hyperactivity and impulsivity (I/HI), core characteristics of attention-deficit/hyperactivity disorder (ADHD), are among the most frequent psychiatric comorbidities in Autism Spectrum Disorder (ASD). Such comorbidity further compromises functioning and poses challenges for treatment. Yet, little is known regarding the mechanisms underlying the co-occurrence of ASD and I/HI symptoms. For example, it remains unclear if I/HI symptoms in ASD stem from distinct mechanism(s) from those in ADHD and whether assessment and treatment protocols should be identical. Research aiming to attain such knowledge can help advance neuroscientific models of ASD heterogeneity. Further, this work represents a critical step towards identifying individuals at greater risk of I/HI comorbidity, those more likely to respond to currently available treatments, and informing treatment development. This panel integrates state-of-the-art clinical and brain imaging research aimed to provide a nuanced understanding of the overlap of I/HI and ASD. In presenting empirical findings, we will highlight the processes of identifying clinically meaningful markers able to ‘stratify’ children with ASD and I/HI. The working framework that we will present can inform a range of other efforts addressing sources of heterogeneity in ASD related to psychiatric comorbidities.