The X-Ray CT scanner and MRI imaging of the brain have changed the study of this organ, in health and in disease. To add to the detailed demonstration of anatomy, functional imaging of the human brain is now carried out by either positron emission computed tomography (PET) or by single photon emission computed tomography (SPET). PET allows for the investigation of brain blood flow, metabolism and many neuroreceptors and is well placed to study the interaction between a drug and the brain. Its clinical (patient-orientated) problem solving potential remains limited--it is too costly a methodology. SPET of the human brain is the affordable, patient-dedicated, alternative. SPET is available in hundreds of departments throughout the U.K. and the E.E.C. Significant progress has been achieved with 99mTc and or 123I-labelled radiopharmaceuticals. Flow studies can be obtained with almost as much detail as most PET flow studies, and progress is at hand with the mapping of neuroreceptor ligands. A metabolic marker for SPET studies is still missing. However flow and metabolism follow a parallel course in most clinically relevant entities (for instance in epilepsy and dementia) -- hence SPET flow maps appear as an attractive alternative to PET. SPET studies of transit (flow/volume) offer an alternative to PET studies of oxygen extraction, since these parameters appear to correlate with each other.