Positron emission tomography (PET) is a quantitative imaging method that can be used to characterize binding properties of specific target molecules such as various receptors, transporter molecules and enzymes in vivo. Although already applied successfully, one of the greatest challenges for the technique is to understand better the in vivo complexities of ligand-receptor (target) interaction. The PET technique can be used efficiently in animal studies but, most importantly, also in human studies. PET imaging of patients and healthy volunteers can generate information on human pathophysiology at a molecular level currently unobtainable with other methods. Modern imaging techniques are increasingly applied to drug discovery and development. There are many ways of utilizing PET in pharmacodynamic studies, one interesting approach being the indirect exploration of synaptic neurotransmission with receptor ligands. The receptor occupancy-type studies with PET are rapidly becoming a state-of-the-art method for verifying the mechanism of action of a given drug in man and especially for facilitating the dose-finding procedures in early drug development. Thus far, PET has been mainly applied to pharmacodynamic studies in the central nervous system but will be used also in other areas of drug development such as cardiovascular diseases and oncology.