Parkinson’s disease (PD) is a neurodegenerative disorder of unknown etiology, not only characterized by motor signs but also by non-motor symptoms, including neuropsychiatric and cognitive dysfunction. The results obtained in the last decades show that the cognitive changes in PD are heterogeneous; impairment in different cognitive domains such as attention, executive, language, memory, and visuospatial functions can be present even in the early stages of the disease. Mild cognitive impairment is frequent in non-demented PD patients and is considered as a risk factor for the development of dementia. As a response to the heterogeneity of cognitive impairment associated with PD, the Movement Disorders Society has recently developed formal diagnostic criteria for mild cognitive impairment and dementia associated with PD. In the present chapter, the authors have conducted a revision of cognitive impairment in PD, describing the results obtained in numerous investigations, from the first studies in the1970s to the advances of the last few years.