For decades, awareness of Parkinson's disease mainly focused on the presence and treatment of motor symptoms. More and more other symptoms of this disease not related to the motor system now receive increased attention, including cognitive decline. Dementia in Parkinson's disease occurs more often than initially was thought. After an average of 8 years, about 40% of these patients suffer from cognitive decline. The difficulties and related problems affect prognosis more than do the motor symptoms. There is a need for standardized diagnostic and specific therapeutic intervention, but appropriate studies are still lacking.