Theories of neurocognitive aging rely heavily on functional magnetic resonance imaging (fMRI) to test hypotheses regarding the brain basis of age-differences in cognition. This technique is based on the blood-oxygen level dependent signal (BOLD) that arises from the coordinated neural-vascular coupling that leads to increased blood flow following an increase in neural activity. Here we review the literature and current controversies regarding the mechanisms by which blood flow and neural activity are coupled, and how they change in the aging process. This literature suggests that neural-vascular coupling is a complex of processes, involving dynamic signaling between neurons, glia, and blood vessels. Nearly every component of this process is affected in aging leading to changes in BOLD and pervasive age-related cognitive changes.