Parkinson's disease is the second most frequent neurodegenerative disorder. There is significantly elevated risk of cognitive decline and associated neuropsychiatric symptoms. Dementia may develop insidiously several years after manifestation of Parkinson motor symptoms (dementia associated with Parkinson's disease; Parkinson's disease dementia) or in close temporal relationship (within one year) after onset of motor symptoms (Dementia with Lewy bodies). There are clinical, pathophysiological and therapeutic similarities between these two conditions. Men are more frequently affected than women. Risk factor or indicators are advanced age at disease onset, disease duration, rigidity, akinesia and posture and gait impairment and falls as opposed to tremor dominance, and associated neuropsychiatric symptoms (depression, apathy, hallucinosis, delirium). Dementia is treatable with cholinesterase inhibitors (rivastigmine, donepezil), memantine, and adjustment of the pharmacological regimen of parkinsonian motor symptoms. Concomitant autonomic nervous system symptoms and neuropsychiatric complications warrant early clinical awareness and are accessible to pharmacological therapy.