The smartphone represents a transformative device which dramatically changed our daily lives, including how we communicate, work, entertain us, and navigate through unknown territory. Given its ubiquitous availability and impact on nearly every aspect of life, debates on the potential impact of smartphone (over-)use on the brain and if smartphone use can be “addictive” have increased over the last years. During the last years the number of studies that employed magnetic resonance imaging (MRI) to characterize associations between individual differences in excessive smartphone use and variations in brain structure and function has strongly increased and it is now an opportune time to summarize and critically reflect upon the available studies. Following this overview, we present a roadmap for future research to improve our understanding of how excessive smartphone use can impact upon the brain, mental health, and cognitive and affective functions. Keywords: smartphone, smartphone use disorder, problematic smartphone use, smartphone addiction, internet, internet addiction, MRI, review, brain, fMRI, mental health