Prevalence of dementia continues to increase because of the aging population and limited treatment options. Cerebral small vessel disease and Alzheimer disease are the two most common causes of dementia with vascular dysfunction being a large component of both their pathophysiologies. The neurogliovascular unit, in particular the blood-brain barrier (BBB), is required for maintaining brain homeostasis. A complex interaction exists among the endothelial cells, which line the blood vessels and pericytes, which surround them in the neurogliovascular unit. Disruption of the BBB in dementia precipitates cognitive decline. This review highlights how dysfunction of the endothelial-pericyte crosstalk contributes to dementia, and focuses on cerebral small vessel disease and Alzheimer disease. It also examines loss of pericyte coverage and subsequent downstream changes. Furthermore, it examines how disruption of the intimate crosstalk between endothelial cells and pericytes leads to alterations in cerebral blood flow, transcription, neuroinflammation, and transcytosis, contributing to breakdown of the BBB. Finally, this review illustrates how cumulation of loss of endothelial-pericyte crosstalk is a major driving force in dementia pathology.