Receptor (or enzyme) Occupancy (RO) is a quantitative measure of what percentage of the total number of available target receptors is engaged by or bound to a particular ligand. In drug discovery, it is important to determine the relationship of a receptor’s occupancy to its final observed pharmacological efficacy and/or toxicological readout. RO is related to but distinct from a ligand’s potency and maximal efficacy. Different compounds may achieve different percentages and durations of occupancy. Receptor occupancy/exposure relationships as well as the duration of occupancy have been used to project clinical doses.