Study on the effects of children’s screen time activity on their mental health and brain structure The generation of ''digital natives'', born between the end of the last century and the beginning of the current one, has been growing up surrounded by a screen in every part of their lives: screens are used to study, for entertainment, and social activities. Children in the age 8-18 spend, on average, 7.5 hours every day in front of a screen, 4.5 of which are spent watching TV. The effects of screen time activity on their mental health and brain structure are still not fully understood, and evidence for an impact of screentime on the health is inconsistent, with systematic reviews showing mixed findings. This may in part be due to the difficulty of separating the effects of screentime from other consequences of sedentary life, like low physical activity or poor sleep quality. Moreover, most of the literature is based on cross-sectional data, therefore it struggles to provide evidence on causal association or the direction of the relationships between screentime activity, mental health, and brain structure. This thesis aims to examine these relationships through the use of statistical models fitted on data from Adolescent Brain Cognitive Development longitudinal study, a US representative sample of more than 11000 children in age 8-12. The screentime activity is divided into four main types: TV, video (e.g., YouTube), videogames, and social activities. Mental health is evaluated through the use of psychometric scales obtained from Children Behavior Checklist, whereas brain structure is described by anatomical measures like intracranial volume (ICV), cortical thickness, and cortical surface area, extracted through anatomical MRI (T1w) of the subjects' brain. The analyses showed evidence of a positive association between time spent watching TV and the scores concerning externalizing problems, like rule-breaking and aggressive behavior, whereas social activities have a negative association with internalizing problems like anxiety and withdrawal. On the other hand, time spent playing video games proved to have a significant effect on the thinning of the cerebral cortex, especially in some temporal regions.