Several techniques can be used to measure indirectly the effect of drugs (e.g., EEG, fMRI) in healthy volunteers and in patients. Although each technique has its merits, a direct link between drug efficacy and site of action in vivo usually cannot be established. In addition, when the specific mode of action of a drug has been determined from preclinical studies, it is often not known whether the administered dose is optimal for humans. Both industry and academia are becoming more and more interested in determining the dose-related occupancy of specific targets caused by administration of drugs under test. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are noninvasive imaging techniques that can give insight into the relationship between target occupancy and drug efficacy, provided a suitable radioligand is available. Although SPECT has certain advantages (e.g., a long half-life of the radionuclides), the spatial and temporal resolution as well as the labeling possibilities of this technique are limited. This review focuses on PET methodology for conducting drug occupancy studies in humans.