Alzheimer’s disease (AD) is the most common form of dementia among older people. The blood-brain barrier (BBB) is a highly selective permeable barrier that separates the brain from circulating blood. It is formed by brain endothelial cells and prevents harmful materials from the blood from entering the brain. Evidence identifying BBB dysfunction in AD or patients at risk (i.e., those with mild cognitive impairment) continues to escalate. In cerebral amyloid angiopathy (CAA), which is a unique form of AD, a toxic molecule generated in AD brain, called Abeta, deposits within the blood vessels of the brain. The deposition of Abeta leads to BBB impairment, including microhemorrhages, which contribute to AD pathogenesis in CAA. However, little is known about the role of the BBB function in AD pathogenesis. We propose to elucidate the function of BBB in AD progression and investigate whether toxic molecules generated in AD brain cause BBB impairment.