Functional imaging methods exploit the relationship between neuronal activity, energy demand and cerebral blood flow to functionally map the brain. Despite the increasing use of these imaging tools in basic and clinical neuroscience, the neurobiological processes underlying the imaging signals remain unclear. Recently, interest has been focused on uncovering the signals that trigger the metabolic and vascular changes accompanying variations in neuronal activity. Advances in this field have demonstrated that release of the major excitatory neurotransmitter glutamate initiates diverse signaling processes between neurons and astrocytes, and that this signaling could be crucial for the occurrence of brain imaging signals. In this article we review the hypothesis that glutamate represents a common trigger for both neurometabolic and neurovascular coupling.