It is important to keep in mind that individual brain imaging methods typically address only one aspect, or at best a few aspects, of underlying cerebral physiology and pathophysiology (Table 1). For example, electromagnetic physiologic techniques, such as electroencephalography (EEG), event-related potentials, and magnetoencephalography, provide information about large constellations of neurons and the net electromagnetic physiologic vector that their activity produces. These methods have the best temporal resolution currently available but are lacking in regard to spatial resolution and localization accuracy. Other techniques are devoted specifically to measurements of brain structure. These include x-ray computed tomography (CT), conventional magnetic resonance imaging (MRI), and blood vessel imaging using conventional angiography, magnetic resonance angiography, or helical CT.7 A third group of methods assesses hemodynamic response as a measure of brain function. Techniques in this group include xenon-enhanced CT, functional MRI (fMRI), perfusion MRI (pMRI), and cerebral blood flow or volume measurements obtained with either positron emission tomography (PET) or single photon emission computed tomography (SPECT). These methods assume that neuronal firing and blood flow increments or decrements are tightly coupled. This seems reasonable in the normal brain, but it may not always be true in pathologic states. Methods that determine the chemical processes of the brain fall mainly in the domain of PET and magnetic resonance spectroscopy. PET is able to measure glucose metabolism, protein synthesis, amino acid uptake, pH, and other variables in a quantitative manner with results calculated in the appropriate physiologic units. Magnetic resonance spectroscopy provides relative measurements of chemical compounds typically derived from hydrogen proton spectra, but, at higher magnetic field strengths, it is also possible to estimate relative quantities of molecules containing resonant isotopes of sodium, fluorine, carbon, and phosphorus. Imaging of brain receptor systems, both neurotransmitter molecules and receptor complexes, has been an active area of research in both health and disease using PET and SPECT. Most information has been derived for the dopaminergic system, but data also exist for the cholinergic, serotonergic, opioid, and benzodiazepine systems.