Positron emission tomography (PET) is a noninvasive radiotracer-based technique which increasingly is being applied to describe the functional anatomy of the human brain in life. It is a technically sophisticated approach to perfusion mapping, and is predicated on the fact that increases and decreases of synaptic activity in the brain are accompanied by appropriate and equivalent changes in local glucose consumption and perfusion (Raichle, 1987; Mata et al. 1980; Fox and Raichle, 1986). The achievable, practical resolution of the scans presently approximates 6 x 6 x 6 mm, which is sufficient to identify focal perfusion changes as little as 2 mm apart if sequential bloodflow maps are compared and hence to permit analysis of functional activation in the brain at the level of maps, networks and systems. It is theoretically possible that technical advances will one day allow some resolution at a cortical modular level. The tracer of perfusion most commonly used is water, labelled with radioactive, positron-emitting oxygen (15O), which has a short 2.1 min half-life. There is some interest in using 15O labelled butanol which has, in theory, certain possible advantages over water as a perfusion tracer. 15O-water can be used to record up to 12 estimations of the distribution of cerebral perfusion at one sitting in normal subjects and is very easy to use. The resultant radiation dose is very small, safe and within international guidelines for the use of radioactivity for research in normal human volunteers (5 mSv).