The prevalence of cognitive impairment and dementia in Parkinson's disease (PD) is high and can potentially occur as the result of multiple differing pathologies. Neuroimaging has provided evidence of decreased cortical volume, increased white matter diffusion changes, and decreased resting metabolic activity that appears to begin prior to the onset of dementia in PD patients. Cognitive impairment has been found to be associated with multiple neurotransmitter transmission deficiencies, including dopamine and acetylcholine, indicating a widespread neurotransmitter dysfunction in PD-related dementia. Findings of increased Pittsburgh Compound B (PiB) binding in subjects with Lewy Body Disease (LBD) compared with Parkinson's disease and dementia (PDD) may explain phenotype differences in the spectrum of Dementia with Lewy Bodies (DLB), and show promise in guiding future therapeutic trials aimed at this disease. Advances in neuroimaging now allow for the detection of volumetric, pharmacologic, and pathological changes that may assist in the diagnosis and prediction of cognitive impairment in Parkinson's patients so that better evaluation of disease progression and treatment can be obtained.