Cognitive impairment represents an important and often defining component of the clinical syndromes of Lewy body disorders: Parkinson’s disease and dementia with Lewy bodies. The spectrum of cognitive deficits in these Lewy body diseases encompasses a broad range of clinical features, severity of impairment, and timing of presentation. Cognitive dysfunction is now recognized to occur not only in more advanced Parkinson’s disease, but also in early, untreated patients, and even in those patients with pre-motor syndromes such as REM behavior disorder and hyposmia. In recent years, the concept of “mild cognitive impairment” as a transitional or pre-dementia state in Parkinson’s disease has emerged. While this has led to much research regarding the diagnosis, prognosis, and underlying neurobiology of mild cognitive impairment in Parkinson’s disease, it has also raised questions regarding the usefulness of this concept and its application in clinical and research settings. In addition, the conundrum of whether Parkinson’s disease dementia and dementia with Lewy bodies represent the same or different entities remains unresolved. While these disorders overlap in many aspects of their presentations and pathophysiology, they differ in other aspects such as timing of cognitive, behavioral, and motor symptoms, medication responses, and neuropathological contributions. This article examines the spectrum and evolution of cognitive impairment in Lewy body disorders and debates these controversial issues in the field using point-counterpoint approaches.