Enzyme-linked immunosorbent assays (ELISAs) can be designed to detect either antigens or antibodies. Nearly all ELISA formats require the separation of reactants from the products of the immunoassay. The product of the assay is an immune complex consisting of target ligand, the analyte, and the reporting probe used to detect the complex that is formed. Because it 1s usually necessary to remove excess analyte as well as excess probe before measurements, several separation steps may be desired, if not required. In the early years, during the evolution of radioimmunoassays (RIAs), it was found that, under certain conditions, antibodies or antigens would bind strongly during brief exposures of solutions to plastic surfaces, such as the inner walls of a tube or a well. The amount of target ligand that would bind proved to be well suited for the competitive assays that were being developed then; the convenience that this phenomen provided has spurred the growth of immunoassay systems for 25 years. Although ELISAs have now generally replaced most RIAs, the principles for coating of the solid phases are similar for both.