Dementia with Lewy bodies (DLB) and Parkinson's disease dementia (PDD) share common clinical, neuropsychological and pathological features. In clinical diagnosis, distinguishing between these conditions and other dementia subtypes such as Alzheimer's disease (AD) can be difficult. Despite the development of consensus diagnostic criteria, sensitivity for diagnosis remains low, especially outside specialist centres. Neuroimaging techniques using magnetic resonance (MR) can assess changes in structure, microstructure through diffusion tensor imaging and metabolism using spectroscopy and cerebral perfusion. Identification of such changes may contribute to our understanding of the disease process, assist in refining ante-mortem diagnosis and allow disease progression to be measured. This may be both clinically useful and a tool for assessing outcome in therapeutic trials. DLB and PDD share a similar pattern of MRI changes including global brain volume loss, a predominantly subcortical pattern of cerebral atrophy and structural preservation of the medial temporal lobe compared to AD. This review summarises the application and findings from MR studies in DLB and PDD to provide further insight into the similarities between the conditions, highlight the potential for the clinical application of MR techniques and outline promising areas for further research.