What percentage of receptors, ion channels or transporters must be occupied by drugs to trigger therapeutic effects in patients, or by drugs and other ligands to induce physiological effects in humans or animals? Human studies utilizing Positron Emission Tomography and Single Photon Emission Computed Tomography, along with data from an array of preclinical methodologies, have begun to provide consistent answers to this question. The required target occupancy is dependent upon the molecular class of both target and ligand, and appears to be similar for both patient therapy and human or animal physiology. In the case of antagonists, approximately 60–90% target occupancy is required for G protein-coupled receptors, neurotransmitter transporters, and ligand-gated ion channels. Effective doses of agonists occupy a wider range of their target sites, dependent upon the intrinsic activity of the agonist, the receptor or ion channel reserve of the target site, and the response that is measured, with low efficacy agonists generally requiring high degrees of occupancy while high efficacy agonists generally require low degrees of occupancy. Target desensitization, competition by endogenous ligands, and regional target differences all influence target occupancy requirements. Measurements of target occupancy can help assure proper dosing and targeting of compounds in preclinical and clinical drug development as well as in basic research. Target occupancy generalizations can be especially important in establishing initial dosing recommendations for the many new drug targets provided by genomic and proteomic initiatives, where little data is available on their functional responses.