This chapter reviews relevant magnetic resonance imaging (MRI) methods and their application to various stages of disease including early or incident Parkinson's disease (PD), treated patients, including those with mild cognitive impairment (MCI), and those with Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB). In PD pathological changes have been found in the hippocampus, parahippocampus, amygdala, and the entorhinal cortex. Several groups of investigators have shown that the substantia innominata is atrophic in PD, with progressive loss in PD-MCI and PDD. The chapter also reviews longitudinal studies and the impact of white matter pathology. A study that compared progression of white matter changes between Alzheimer's disease (AD), PDD, and DLB and older controls found that AD patients had more white matter changes than controls; there was no difference in progression of white matter changes between the dementia groups.