This chapter reviews Parkinson’s disease dementia, Lewy body dementia and Parkinson’s plus disorders such as multiple systems atrophy, progressive supranuclear palsy, and cortico-basal degeneration. Incidence and prevalence figures are reviewed, along with the pathological mechanisms underlying these disorders. The difference between Parkinsons’s dementia and Lewy body dementia is considered, along with a review of the diagnostic criteria for each (including Parkinson’s disease mild cognitive impairment). Neuropsychological profiles are considered, although other differences in presentation of these disorders may be more important for diagnosis. Although rare, and more likely to present to neurology services, the diagnostic criteria for Parkinson’s plus disorders are also considered.