The current thesis focused on the effects of physical activity on neurocognitive functioning and brain properties, and had two main aims. The first aim was to explore the relation between physical activity and neurocognitive functioning in healthy preadolescent children, including the possible role of brain structure and function in this relation. To address this aim, cross-sectional studies were performed in which two derivatives pf physical activity were studied; cardiovascular fitness and gross motor skills (PART I). The second aim of this thesis was to investigate the effects of physical activity on brain structure, brain functioning and neurocognitive functioning in children through literature and experimental studies. Furthermore, the potentially differential effects of aerobic and cognitively demanding exercise were investigated (PART II). The results of PART I indicate relations between both cardiovascular fitness and gross motor skills with brain structure, functioning and neurocognitive functioning in healthy preadolescent children. Furthermore, a mediating role in the relation between aspects of physical activity and neurocognitive functioning was found for brain functioning (RSN brain activity), while no mediating role was found for brain structure (white matter microstructure) within this relation. The findings of PART I support the idea that improving cardiovascular fitness and/or gross motor skills during childhood may represent a means to induce reorganization of brain structure and functioning, which in turn may benefit neurocognitive functioning in childhood. These findings highlight the relevance of physical activity for brain development. The results of PART II indicate that, based on a systematic review and meta-analysis, both short-term and long-term physical exercise may induce changes in neurophysiological functioning in both healthy children and clinical populations, while no evidence was found for effects of physical exercise on brain structure in children. Nevertheless, in our experimental studies, we did not find evidence for effects of 14-week school-based aerobic or cognitive-demanding physical exercise interventions on neurocognitive functioning, brain structure or brain function in healthy preadolescent children. This thesis provides evidence for positive relations between aspects of physical activity and brain structure, brain function and neurocognitive functioning. Furthermore, a mediating role was found for brain functioning within this relation. These results support the idea that exposure to physical activity during childhood facilitates neuroplasticity and neurocognitive functioning during childhood. Despite the cross-sectional and meta- analytic evidence for effects of physical activity, our RCT did not provide evidence for the effects physical activity on brain structure or functioning in children. The 14-week aerobic and cognitively demanding exercise interventions did not elicit changes in brain structure or functioning and thereby neurocognitive functioning in children. However, more time spent in MVPA was associated with better neurocognitive functioning and spending additional time on physical activity had no adverse effects on children’s cognitive performance and academic achievement. The current increase in sedentary behavior among children in the context of the promising results on the positive relationship between physical activity and neurocognitive functioning emphasize the importance of physical activity during childhood