It took almost a century and several discoveries in the seemingly unrelated field of quantum physics to allow researchers to be able to use changes in blood flow and volume to identify areas of neural activity. The most widely used techniques to do so include positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). In addition to measuring task-induced changes in cerebral blood flow (CBF) or cerebral metabolism, PET imaging can be used to directly and selectively assess the action of different neurotransmitters in the human brain in vivo. The change in the BOLD signal triggered by a brief neural event is known as the hemodynamic response (HDR). It is important to keep in mind that, as is the case with any experimental method, there are limitations and potential pitfalls that one needs to consider when designing, analyzing, or interpreting experiments using PET or fMRI.