There is a growing body of evidence suggesting that Alzheimer's disease (AD) and cerebrovascular disease (CVD), two common disorders of aging, are related in ways that extend beyond their coincident co-occurrence. Thus, there is a need to understand how the two pathological processes interact to contribute to dementia. In this article, we review two recent studies from our laboratory that use MRI and PET to begin to reveal the nature and mechanisms of the relationship between AD, CVD, and dementia.