Disturbances of cognition are frequent in Parkinson's disease (PD). Unlike severe loss of dopamine early in PD, extensive cholinergic losses have been consistently reported in PD with dementia. Cholinergic imaging suggests that basal forebrain cholinergic system degeneration appears early in PD and worsens with dementia development. Cortical cholinergic denervation is similar in PD with dementia and dementia with Lewy bodies, supporting a common disease spectrum, at least with respect to cholinergic pathology. Presence of cerebral amyloidopathy in the setting of parkinsonism may accelerate cognitive decline. Novel MRI techniques illustrate the widespread presence of neurodegeneration in PD with dementia, affecting white matter tracts and connectivity functions. This review will outline current concepts regarding dementia development in PD and discuss their correlation with functional and structural neuroimaging including PET and MRI.