Opiate receptors in the brain are the target of endogenous opioids and of exogenous synthetic opiates. It is well established that these receptors play a major role in the modulation of pain perception. With positron emission tomography (PET) and the appropriate radioligands, it is now possible to image and quantify neuroreceptors in vivo. We used 11C-diprenorphine and the extremely potent mu opiate receptor agonist 11C-carfentanil to image the distribution of opiate receptors in the human brain and to quantify their density, affinity, and occupancy. Several important methodological aspects of the in vivo opiate receptor labeling with PET in relation to the study of pain are considered in this paper. Monitoring receptor occupancy by opiate drugs as a function of pain relief has the potential to reveal better ways to treat pain.