Recent progress in brain imaging offers great potential for anaesthesia research. Among the most important imaging modalities for mapping human brain activation are positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Both techniques are based on mapping haemodynamic or metabolic changes that are a direct consequence of alterations in neuronal activity. 59 In comparison with electroencephalographic techniques, which are able to demonstrate that a given anaesthetic agent has central nervous system activity, PET or fMRI provide information about the pathways and the anatomical localization of the drug effect. On one hand, fMRI offers a superior spatial resolution to PET. The theoretical limit for PET is expected to be 2 mm isotropic resolution for the human head, whereas a recent fMRI study has demonstrated an in-plane resolution of 0.55 mm. Furthermore, fMRI does not require the injection of exogenous radioactive tracers. Therefore, a subject can perform a variety of different tasks during various experimental conditions within the same imaging session. Besides these methodological factors, fMRI is of advantage in terms of lower costs and clinical availability. On the other hand, PET offers the unique possibility of in vivo receptor imaging, only shared in part with magnetic resonance spectroscopy, and it provides absolute values of physiological variables. Using the complementary features of both techniques allows us to obtain information about how the brain works from the molecular to the complex neural network level and to explore regional drug effects on integrated brain processes at these various levels. Both imaging modalities have the ability to reveal new insights in altered brain processes during anaesthesia and may contribute to our understanding of where, why, and how brain functions collapse in the presence of anaesthetic drugs. This review will focus mainly on previously reported results obtained with human brain imaging techniques related to the general mechanism of anaesthetic action. It will ®rst discuss some methodological aspects of PET and fMRI and then comment on results obtained mostly from volunteer studies.