Diabetes mellitus is associated with an elevated risk of cognitive impairment and dementia. Cerebrovascular disease and neurodegeneration are two major pathways that may explain the effect of diabetes on the brain and therefore deserve investigation. Neuroimaging provides an effective way to investigate the contribution of these pathways in vivo, guiding further mechanistic research and providing biomarkers for clinical correlation or interventional studies. In this paper, we present a narrative review of the state of play with neuroimaging evidence in studies of people with diabetes mellitus, how these data are useful in understanding mechanistic links between diabetes and brain impairment, and possible ways that the field may develop in the future.