Radioactive tracers have made an immense contribution to the understanding of human physiology and pathology. At the start of the 21st century nuclear imaging has emerged as the main metabolic imaging modality which is of growing importance in drug development and clinical pharmacology. Using techniques adapted from those undertaken in clinical radiopharmacy and nuclear medicine facilities drug molecules and carrier systems may be radiolabelled and their release, biodistribution and uptake may be visualized in human subjects. Imaging studies are capable of locating the uptake of specific receptors in the brain, the site of disintegration of a tablet in the GI tract, the penetration of a nebulized solution into the lung and the residence time of an eye drop on the cornea. The technology uses suitable gamma emitting radionuclides such as 99mTc, 111In, 123I and 153Sm, which may be imaged with a gamma camera or positron emitters such as 11C, 13N, 15O and 18F for positron emission tomography (PET). Positron emitters are more appropriate for the direct labeling of drug molecules rather than metals such a 99mTc or 111In. A particular asset of these techniques is that the in vivo distribution and kinetics of a radiolabelled pharmaceutical formulation may be quantified. In this way correlation between the observed pharmacological effects and the precise site of delivery may be made. A powerful feature of nuclear molecular imaging is the evaluation of drug delivery systems in patient groups for whom the treatment is intended. Such studies not only provide data on the nature and characteristics of a product, such as reliability and reproducibility, but can demonstrate proof of principle for the new generation of targeted therapeutics. Imaging data are increasingly being used in product registration dossiers for submission to Regulatory Authorities.