Type 2 diabetes (T2D) has been associated with dementia in countless observational epidemiology studies. The expansion of epidemiologic research on T2D and dementia is due to scientific recognition of the roles of metabolic and vascular factors as etiologic players in dementia, as well as ominous global demographic shifts in aging, obesity, and dementia. This chapter addresses epidemiologic studies evaluating the association between T2D and late-onset dementias with foci on (1) T2D and dementia as syndromes; (2) T2D and mild cognitive impairment or cognition and cognitive decline; (3) vascular and metabolic risk factors and comorbidities; (4) genetic influences on the T2D-dementia association; (5) ethnoracial considerations; (6) T2D and brain outcomes and biological markers; and (7) clinical trials of T2D medications and cognition and dementia. © 2018 Elsevier Inc. All rights reserved.