Dementia in patients suffering from Parkinson’s disease (PD) has long been considered unrelated to the disease. The constant improvement in the management of motor symptoms and the consequent increase in the life expectancy of these patients brought out cognitive impairment as an intrinsic feature of PD that in severe stages may dominate the clinical picture. Certain clinical characteristics such as the age and the motor subtype of the disease may increase the risk of dementia. At least at its early stages, PD-related cognitive impairment follows a subcortical-dysexecutive pattern, with additional cortical deficits frequently characterizing the transition to dementia. Dysfunction in both dopaminergic and non-dopaminergic networks has been implicated in the pathogenesis of cognitive decline in PD. At cellular level the spreading of Lewy bodies/neurites constitutes the fundamental pathology, while Alzheimer’s disease-related protein aggregates are also detected. The assessment of cognitive decline may be quite challenging and requires specific neuropsychological instruments. PD-specific cognitive scales, suitable for both short and detailed examination, have been developed. Criteria for the diagnosis of dementia and mild cognitive impairment in PD have been also proposed by expert work groups. Rivastigmine is the only FDA approved treatment for PD dementia until now, but several other treatments are currently under testing.