Autism and Attention-Deficit/Hyperactivity Disorder (ADHD) are highly heterogeneous neurodevelopmental disorders (NDDs) that are associated with attention and executive function (EF) deficits (Neely et al. 2016; Craig et al. 2016). Despite EF and attention being common deficits, groups of individuals with Autism and ADHD are not easily differentiated from each other or from controls by measures of attention and EF (Cai et al., 2021; Kushki et al., 2019). The process for diagnosis of Autism and ADHD is long and requires considerable resources. To make this process more efficient, a better understanding is required about the types of assessments and cognitive domains that contribute more or less to diagnostic status (which will be explored in Chapter 2). The search for neural correlates of Autism and ADHD, such as differences in functional connectivity (FC) related to diagnostic status, has resulted in highly inconsistent results (Cortese et al., 2021; Liloia et al., 2024). This could, in part, be due to the heterogeneity of symptom severity and presentation in Autism and ADHD (Luo et al. 2019; Dajani et al. 2016). By assessing the neural correlates of specific attention and EF measures transdiagnostically, there may be more consistency in results (Mooney et al., 2024). In the search for neural correlates it is also important to acknowledge the factors that could also influence the brain-behaviour relationship, such as stimulant medication (Schweren et al., 2016; Swanson et al., 2011) (further explored in Chapter 3), or diagnostic status (Zhang et al. 2024) (explored in Chapter 4). The lack of consideration for these factors could also contribute to a lack of convergence in results, thus it is important to understand the factors that could influence the brain-behaviour relationship.