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On a daily basis, clinicians bridge the gap between scientific knowledge of a diagnostic category on the one hand; and valuable anecdotal knowledge about a single child and all his distinctive characteristics on the other. Combining these sources of information is the backbone of clinical decision making. If neurobiological models of ADHD can incorporate individual differences between children with the same diagnosis, they may become useful to help guide clinical practice. However, to date, most of our knowledge on the neurobiology of ADHD comes from studies that have focused on differences at the group level. In this thesis, we aimed to further our knowledge and move in the direction of clinical applicability by exploring possible pitfalls of this between-group approach. We used fMRI to study different neuropsychological domains that have been implicated in the neurobiology of ADHD: cognitive control, reward sensitivity and timing. In addition, in three different studies included in this thesis, we added a third group of children: children with a primary diagnosis of ASD and parent-rated symptom levels of ADHD, in addition to children with ADHD and typically developing children. This was done to achieve a more trans-diagnostic approach of the underlying neurobiology. Moreover, we used latent class analysis (LCA) to identify subgroups among children with attention problems on the basis of task performance or brain activity. In a series of six papers on individual differences in the neurobiology of children with ADHD, we came to the following conclusions: - As a group, children with ADHD are affected in multiple neuro-psychological domains: reward sensitivity, timing and cognitive control. - Problems with response inhibition that have been consistently reported in ADHD are related to reactive inhibition; we found no evidence for problems with proactive inhibition. - The neurobiological changes reported in this thesis are mostly not specific to ADHD, but are also found in children with ASD and similar levels of parent-rated ADHD symptoms. - Within a group of children with symptoms of ADHD, there is no clear-cut linear relation between ADHD-like behavior and changes in brain activity or neurocognitive performance. - Within a group of children with symptoms of ADHD, reward sensitivity in daily life is positively associated with brain activity during reward anticipation. - Within a group of children with symptoms of ADHD, different subgroups of children can be identified based on distinct neuropsychological and neurobiological profiles. In this thesis, we explored different ways to move away from solely studying between-group differences and towards neurobiological models of ADHD that incorporate individual differences. At this point in time, studying developmental disorders is not about constructing a perfect biophysical model that explains all psychological functioning. It is about choosing models of underlying biology that work best; that help us understand behavior a little better, and in turn help us improve our predictions of what will and what will not work for an individual child.