Cognitive impairment is common in Parkinson's disease (PD) and involves attentional, executive, visuospatial, and memory dysfunctions. Dementia is more frequently encountered in PD than in age-matched control populations, and whereas operational definitions of Alzheimer's disease and dementia with Lewy bodies have been developed, Parkinson's disease dementia (PD-D) has remained undefined. The Movement Disorder Society developed a task force to define and develop diagnostic guidelines for PD-D. This effort was based on existing descriptive studies with special emphasis on drawing distinction among Alzheimer's disease, dementia with Lewy bodies and PD-related cognitive impairment without dementia whenever possible. The second goal was to provide practical diagnostic procedures to diagnose PD-D. This effort emphasized available bedside tools that do not require neuropsychological expertise to administer or interpret. This work recently has been completed, and two primary articles have been published. The suggested clinical diagnostic criteria for PD-D involve four domains and are anchored in core features, associated clinical features, features that make the diagnosis uncertain, and features that are not compatible with the diagnosis of PD-D. When all four criteria are satisfactorily met, probable PD-D is designated; when the first and last criteria are met, but clinical characteristics are atypical or uncertainty factors exist, possible PD-D is designated. Whereas these definitions are operative and subject to change based on future data, they are based on widely available tests. The inclusion criteria call be applied internationally and in multicenter research on treatment interventions, clinicopathological correlations, and studies of cognitive and other nonmotor elements of PD.